diff --git a/src/ScadaLink.Commons/Interfaces/Repositories/INotificationOutboxRepository.cs b/src/ScadaLink.Commons/Interfaces/Repositories/INotificationOutboxRepository.cs
new file mode 100644
index 0000000..34fa932
--- /dev/null
+++ b/src/ScadaLink.Commons/Interfaces/Repositories/INotificationOutboxRepository.cs
@@ -0,0 +1,57 @@
+using ScadaLink.Commons.Entities.Notifications;
+using ScadaLink.Commons.Types.Notifications;
+
+namespace ScadaLink.Commons.Interfaces.Repositories;
+
+/// <summary>
+/// Data access for the central notification outbox — the queue of <see cref="Notification"/>
+/// rows the outbox actor drains, retries, and audits. Distinct from
+/// <see cref="INotificationRepository"/>, which manages notification list configuration.
+/// </summary>
+public interface INotificationOutboxRepository
+{
+    /// <summary>
+    /// Inserts <paramref name="n"/> only if no row with the same
+    /// <see cref="Notification.NotificationId"/> exists. Returns <c>true</c> when a new
+    /// row was inserted, <c>false</c> when an existing row was left untouched.
+    /// </summary>
+    Task<bool> InsertIfNotExistsAsync(Notification n, CancellationToken ct = default);
+
+    /// <summary>
+    /// Returns notifications ready for a delivery attempt: <c>Pending</c> rows, plus
+    /// <c>Retrying</c> rows whose <c>NextAttemptAt</c> is at or before <paramref name="now"/>.
+    /// Terminal rows are excluded. Ordered by <c>CreatedAt</c> ascending, capped at
+    /// <paramref name="batchSize"/>.
+    /// </summary>
+    Task<IReadOnlyList<Notification>> GetDueAsync(DateTimeOffset now, int batchSize, CancellationToken ct = default);
+
+    /// <summary>Returns the notification with the given id, or <c>null</c>.</summary>
+    Task<Notification?> GetByIdAsync(string notificationId, CancellationToken ct = default);
+
+    /// <summary>Marks <paramref name="n"/> modified and persists it (status transitions).</summary>
+    Task UpdateAsync(Notification n, CancellationToken ct = default);
+
+    /// <summary>
+    /// Returns a page of notifications matching <paramref name="filter"/>, ordered by
+    /// <c>CreatedAt</c> descending, together with the total matching count.
+    /// </summary>
+    Task<(IReadOnlyList<Notification> Rows, int TotalCount)> QueryAsync(
+        NotificationOutboxFilter filter, int pageNumber, int pageSize, CancellationToken ct = default);
+
+    /// <summary>
+    /// Bulk-deletes terminal rows (Delivered/Parked/Discarded) whose <c>CreatedAt</c> is
+    /// older than <paramref name="cutoff"/>. Returns the number of rows deleted.
+    /// </summary>
+    Task<int> DeleteTerminalOlderThanAsync(DateTimeOffset cutoff, CancellationToken ct = default);
+
+    /// <summary>
+    /// Computes a point-in-time <see cref="NotificationKpiSnapshot"/>. The stuck and
+    /// delivered cutoffs are supplied by the caller; the current time used for
+    /// <c>OldestPendingAge</c> is captured inside the method.
+    /// </summary>
+    Task<NotificationKpiSnapshot> ComputeKpisAsync(
+        DateTimeOffset stuckCutoff, DateTimeOffset deliveredSince, CancellationToken ct = default);
+
+    /// <summary>Persists pending changes tracked on the underlying context.</summary>
+    Task<int> SaveChangesAsync(CancellationToken ct = default);
+}
diff --git a/src/ScadaLink.Commons/Types/Notifications/NotificationKpiSnapshot.cs b/src/ScadaLink.Commons/Types/Notifications/NotificationKpiSnapshot.cs
new file mode 100644
index 0000000..4b3af00
--- /dev/null
+++ b/src/ScadaLink.Commons/Types/Notifications/NotificationKpiSnapshot.cs
@@ -0,0 +1,26 @@
+namespace ScadaLink.Commons.Types.Notifications;
+
+/// <summary>
+/// Point-in-time operational metrics for the central notification outbox,
+/// surfaced on the health dashboard.
+/// </summary>
+/// <param name="QueueDepth">Count of non-terminal rows (Pending + Retrying).</param>
+/// <param name="StuckCount">
+/// Count of non-terminal rows (Pending/Retrying) whose <c>CreatedAt</c> is older
+/// than the supplied stuck cutoff.
+/// </param>
+/// <param name="ParkedCount">Count of rows in the Parked status.</param>
+/// <param name="DeliveredLastInterval">
+/// Count of Delivered rows whose <c>DeliveredAt</c> is at or after the supplied
+/// "delivered since" timestamp.
+/// </param>
+/// <param name="OldestPendingAge">
+/// Age of the oldest non-terminal row (<c>now - min(CreatedAt)</c>), or <c>null</c>
+/// when there are no non-terminal rows.
+/// </param>
+public record NotificationKpiSnapshot(
+    int QueueDepth,
+    int StuckCount,
+    int ParkedCount,
+    int DeliveredLastInterval,
+    TimeSpan? OldestPendingAge);
diff --git a/src/ScadaLink.Commons/Types/Notifications/NotificationOutboxFilter.cs b/src/ScadaLink.Commons/Types/Notifications/NotificationOutboxFilter.cs
new file mode 100644
index 0000000..0b405b1
--- /dev/null
+++ b/src/ScadaLink.Commons/Types/Notifications/NotificationOutboxFilter.cs
@@ -0,0 +1,30 @@
+using ScadaLink.Commons.Types.Enums;
+
+namespace ScadaLink.Commons.Types.Notifications;
+
+/// <summary>
+/// Query filter for the central notification outbox. All members are optional;
+/// an unset member means "no constraint on that dimension".
+/// </summary>
+/// <param name="Status">Restrict to a single lifecycle status.</param>
+/// <param name="Type">Restrict to a single delivery channel.</param>
+/// <param name="SourceSiteId">Restrict to notifications originating at a given site.</param>
+/// <param name="ListName">Restrict to a single notification list.</param>
+/// <param name="SubjectKeyword">Substring matched against <c>Subject</c>.</param>
+/// <param name="StuckOnly">
+/// When <c>true</c>, restrict to non-terminal rows (Pending/Retrying) whose
+/// <c>CreatedAt</c> is older than <see cref="StuckCutoff"/>.
+/// </param>
+/// <param name="StuckCutoff">Rows with <c>CreatedAt</c> older than this count as stuck.</param>
+/// <param name="From">Inclusive lower bound on <c>CreatedAt</c>.</param>
+/// <param name="To">Inclusive upper bound on <c>CreatedAt</c>.</param>
+public record NotificationOutboxFilter(
+    NotificationStatus? Status = null,
+    NotificationType? Type = null,
+    string? SourceSiteId = null,
+    string? ListName = null,
+    string? SubjectKeyword = null,
+    bool StuckOnly = false,
+    DateTimeOffset? StuckCutoff = null,
+    DateTimeOffset? From = null,
+    DateTimeOffset? To = null);
diff --git a/src/ScadaLink.ConfigurationDatabase/Repositories/NotificationOutboxRepository.cs b/src/ScadaLink.ConfigurationDatabase/Repositories/NotificationOutboxRepository.cs
new file mode 100644
index 0000000..a9cc6c5
--- /dev/null
+++ b/src/ScadaLink.ConfigurationDatabase/Repositories/NotificationOutboxRepository.cs
@@ -0,0 +1,179 @@
+using Microsoft.EntityFrameworkCore;
+using ScadaLink.Commons.Entities.Notifications;
+using ScadaLink.Commons.Interfaces.Repositories;
+using ScadaLink.Commons.Types.Enums;
+using ScadaLink.Commons.Types.Notifications;
+
+namespace ScadaLink.ConfigurationDatabase.Repositories;
+
+/// <summary>
+/// EF Core data access for the central notification outbox. See
+/// <see cref="INotificationOutboxRepository"/> for the behaviour contract.
+/// </summary>
+public class NotificationOutboxRepository : INotificationOutboxRepository
+{
+    private readonly ScadaLinkDbContext _context;
+
+    // Statuses that represent a finished notification lifecycle. Non-terminal is the complement.
+    private static readonly NotificationStatus[] TerminalStatuses =
+    {
+        NotificationStatus.Delivered,
+        NotificationStatus.Parked,
+        NotificationStatus.Discarded,
+    };
+
+    public NotificationOutboxRepository(ScadaLinkDbContext context)
+    {
+        _context = context ?? throw new ArgumentNullException(nameof(context));
+    }
+
+    public async Task<bool> InsertIfNotExistsAsync(Notification n, CancellationToken ct = default)
+    {
+        var exists = await _context.Notifications
+            .AnyAsync(x => x.NotificationId == n.NotificationId, ct);
+        if (exists)
+        {
+            return false;
+        }
+
+        await _context.Notifications.AddAsync(n, ct);
+        await _context.SaveChangesAsync(ct);
+        return true;
+    }
+
+    public async Task<IReadOnlyList<Notification>> GetDueAsync(
+        DateTimeOffset now, int batchSize, CancellationToken ct = default)
+    {
+        return await _context.Notifications
+            .Where(n => n.Status == NotificationStatus.Pending
+                || (n.Status == NotificationStatus.Retrying
+                    && n.NextAttemptAt != null
+                    && n.NextAttemptAt <= now))
+            .OrderBy(n => n.CreatedAt)
+            .Take(batchSize)
+            .ToListAsync(ct);
+    }
+
+    public async Task<Notification?> GetByIdAsync(string notificationId, CancellationToken ct = default)
+        => await _context.Notifications.FindAsync(new object[] { notificationId }, ct);
+
+    public async Task UpdateAsync(Notification n, CancellationToken ct = default)
+    {
+        _context.Notifications.Update(n);
+        await _context.SaveChangesAsync(ct);
+    }
+
+    public async Task<(IReadOnlyList<Notification> Rows, int TotalCount)> QueryAsync(
+        NotificationOutboxFilter filter, int pageNumber, int pageSize, CancellationToken ct = default)
+    {
+        var query = _context.Notifications.AsQueryable();
+
+        if (filter.Status is { } status)
+        {
+            query = query.Where(n => n.Status == status);
+        }
+
+        if (filter.Type is { } type)
+        {
+            query = query.Where(n => n.Type == type);
+        }
+
+        if (!string.IsNullOrEmpty(filter.SourceSiteId))
+        {
+            query = query.Where(n => n.SourceSiteId == filter.SourceSiteId);
+        }
+
+        if (!string.IsNullOrEmpty(filter.ListName))
+        {
+            query = query.Where(n => n.ListName == filter.ListName);
+        }
+
+        if (!string.IsNullOrEmpty(filter.SubjectKeyword))
+        {
+            query = query.Where(n => n.Subject.Contains(filter.SubjectKeyword));
+        }
+
+        if (filter.StuckOnly && filter.StuckCutoff is { } stuckCutoff)
+        {
+            query = query.Where(n =>
+                (n.Status == NotificationStatus.Pending || n.Status == NotificationStatus.Retrying)
+                && n.CreatedAt < stuckCutoff);
+        }
+
+        if (filter.From is { } from)
+        {
+            query = query.Where(n => n.CreatedAt >= from);
+        }
+
+        if (filter.To is { } to)
+        {
+            query = query.Where(n => n.CreatedAt <= to);
+        }
+
+        var totalCount = await query.CountAsync(ct);
+
+        var rows = await query
+            .OrderByDescending(n => n.CreatedAt)
+            .Skip((pageNumber - 1) * pageSize)
+            .Take(pageSize)
+            .ToListAsync(ct);
+
+        return (rows, totalCount);
+    }
+
+    public async Task<int> DeleteTerminalOlderThanAsync(DateTimeOffset cutoff, CancellationToken ct = default)
+    {
+        return await _context.Notifications
+            .Where(n => TerminalStatuses.Contains(n.Status) && n.CreatedAt < cutoff)
+            .ExecuteDeleteAsync(ct);
+    }
+
+    public async Task<NotificationKpiSnapshot> ComputeKpisAsync(
+        DateTimeOffset stuckCutoff, DateTimeOffset deliveredSince, CancellationToken ct = default)
+    {
+        var now = DateTimeOffset.UtcNow;
+
+        var queueDepth = await _context.Notifications
+            .CountAsync(n => n.Status == NotificationStatus.Pending
+                || n.Status == NotificationStatus.Retrying, ct);
+
+        var stuckCount = await _context.Notifications
+            .CountAsync(n => (n.Status == NotificationStatus.Pending
+                    || n.Status == NotificationStatus.Retrying)
+                && n.CreatedAt < stuckCutoff, ct);
+
+        var parkedCount = await _context.Notifications
+            .CountAsync(n => n.Status == NotificationStatus.Parked, ct);
+
+        var deliveredLastInterval = await _context.Notifications
+            .CountAsync(n => n.Status == NotificationStatus.Delivered
+                && n.DeliveredAt != null
+                && n.DeliveredAt >= deliveredSince, ct);
+
+        // Oldest non-terminal CreatedAt. The DateTimeOffset value converter makes a SQL
+        // Min aggregate awkward, so order ascending and take the first instead.
+        var nonTerminal = _context.Notifications
+            .Where(n => n.Status == NotificationStatus.Pending
+                || n.Status == NotificationStatus.Retrying);
+
+        TimeSpan? oldestPendingAge = null;
+        if (await nonTerminal.AnyAsync(ct))
+        {
+            var oldestCreatedAt = await nonTerminal
+                .OrderBy(n => n.CreatedAt)
+                .Select(n => n.CreatedAt)
+                .FirstAsync(ct);
+            oldestPendingAge = now - oldestCreatedAt;
+        }
+
+        return new NotificationKpiSnapshot(
+            QueueDepth: queueDepth,
+            StuckCount: stuckCount,
+            ParkedCount: parkedCount,
+            DeliveredLastInterval: deliveredLastInterval,
+            OldestPendingAge: oldestPendingAge);
+    }
+
+    public async Task<int> SaveChangesAsync(CancellationToken ct = default)
+        => await _context.SaveChangesAsync(ct);
+}
diff --git a/src/ScadaLink.ConfigurationDatabase/ServiceCollectionExtensions.cs b/src/ScadaLink.ConfigurationDatabase/ServiceCollectionExtensions.cs
index 7aff842..9eca319 100644
--- a/src/ScadaLink.ConfigurationDatabase/ServiceCollectionExtensions.cs
+++ b/src/ScadaLink.ConfigurationDatabase/ServiceCollectionExtensions.cs
@@ -45,6 +45,7 @@ public static class ServiceCollectionExtensions
         services.AddScoped<ISiteRepository, SiteRepository>();
         services.AddScoped<IExternalSystemRepository, ExternalSystemRepository>();
         services.AddScoped<INotificationRepository, NotificationRepository>();
+        services.AddScoped<INotificationOutboxRepository, NotificationOutboxRepository>();
         services.AddScoped<IInboundApiRepository, InboundApiRepository>();
         services.AddScoped<IAuditService, AuditService>();
         services.AddScoped<IInstanceLocator, InstanceLocator>();
diff --git a/tests/ScadaLink.ConfigurationDatabase.Tests/RepositoryCoverageTests.cs b/tests/ScadaLink.ConfigurationDatabase.Tests/RepositoryCoverageTests.cs
index 9de3b56..6f0d11b 100644
--- a/tests/ScadaLink.ConfigurationDatabase.Tests/RepositoryCoverageTests.cs
+++ b/tests/ScadaLink.ConfigurationDatabase.Tests/RepositoryCoverageTests.cs
@@ -6,6 +6,7 @@ using ScadaLink.Commons.Entities.Notifications;
 using ScadaLink.Commons.Entities.Sites;
 using ScadaLink.Commons.Entities.Templates;
 using ScadaLink.Commons.Types.Enums;
+using ScadaLink.Commons.Types.Notifications;
 using ScadaLink.ConfigurationDatabase;
 using ScadaLink.ConfigurationDatabase.Repositories;
 using ScadaLink.ConfigurationDatabase.Services;
@@ -334,6 +335,342 @@ public class NotificationOutboxConfigurationTests : IDisposable
     }
 }
 
+// Coverage for the Notification Outbox repository (Task 5 of the notification-outbox feature).
+public class NotificationOutboxRepositoryTests : IDisposable
+{
+    private readonly ScadaLinkDbContext _context;
+    private readonly NotificationOutboxRepository _repository;
+
+    public NotificationOutboxRepositoryTests()
+    {
+        _context = SqliteTestHelper.CreateInMemoryContext();
+        _repository = new NotificationOutboxRepository(_context);
+    }
+
+    public void Dispose()
+    {
+        _context.Database.CloseConnection();
+        _context.Dispose();
+    }
+
+    private static Notification MakeNotification(
+        string id,
+        NotificationStatus status = NotificationStatus.Pending,
+        DateTimeOffset? createdAt = null,
+        DateTimeOffset? nextAttemptAt = null,
+        DateTimeOffset? deliveredAt = null,
+        string listName = "Ops List",
+        string subject = "Subject",
+        string sourceSiteId = "site-north",
+        NotificationType type = NotificationType.Email)
+    {
+        return new Notification(id, type, listName, subject, "Body", sourceSiteId)
+        {
+            Status = status,
+            CreatedAt = createdAt ?? new DateTimeOffset(2026, 5, 19, 8, 0, 0, TimeSpan.Zero),
+            NextAttemptAt = nextAttemptAt,
+            DeliveredAt = deliveredAt,
+        };
+    }
+
+    [Fact]
+    public async Task InsertIfNotExistsAsync_NewRow_InsertsAndReturnsTrue()
+    {
+        var id = Guid.NewGuid().ToString();
+
+        var inserted = await _repository.InsertIfNotExistsAsync(MakeNotification(id));
+
+        Assert.True(inserted);
+        _context.ChangeTracker.Clear();
+        Assert.NotNull(await _context.Notifications.FindAsync(id));
+    }
+
+    [Fact]
+    public async Task InsertIfNotExistsAsync_DuplicateId_ReturnsFalseAndLeavesExistingRow()
+    {
+        var id = Guid.NewGuid().ToString();
+        await _repository.InsertIfNotExistsAsync(MakeNotification(id, subject: "Original"));
+        _context.ChangeTracker.Clear();
+
+        var inserted = await _repository.InsertIfNotExistsAsync(MakeNotification(id, subject: "Changed"));
+
+        Assert.False(inserted);
+        _context.ChangeTracker.Clear();
+        var loaded = await _context.Notifications.FindAsync(id);
+        Assert.Equal("Original", loaded!.Subject);
+    }
+
+    [Fact]
+    public async Task GetDueAsync_ReturnsPendingAndDueRetrying_OrderedByCreatedAt_CappedAtBatchSize()
+    {
+        var now = new DateTimeOffset(2026, 5, 19, 12, 0, 0, TimeSpan.Zero);
+
+        var pendingOld = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Pending,
+            createdAt: now.AddMinutes(-30));
+        var pendingNew = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Pending,
+            createdAt: now.AddMinutes(-10));
+        var retryingDue = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Retrying,
+            createdAt: now.AddMinutes(-20), nextAttemptAt: now.AddMinutes(-1));
+        var retryingNotDue = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Retrying,
+            createdAt: now.AddMinutes(-25), nextAttemptAt: now.AddMinutes(5));
+        var delivered = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Delivered,
+            createdAt: now.AddMinutes(-40));
+        var parked = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Parked,
+            createdAt: now.AddMinutes(-45));
+
+        _context.Notifications.AddRange(pendingOld, pendingNew, retryingDue, retryingNotDue, delivered, parked);
+        await _context.SaveChangesAsync();
+        _context.ChangeTracker.Clear();
+
+        var due = await _repository.GetDueAsync(now, batchSize: 10);
+
+        // pendingOld, retryingDue, pendingNew are due; ordered by CreatedAt ascending.
+        Assert.Equal(
+            new[] { pendingOld.NotificationId, retryingDue.NotificationId, pendingNew.NotificationId },
+            due.Select(n => n.NotificationId).ToArray());
+
+        var capped = await _repository.GetDueAsync(now, batchSize: 2);
+        Assert.Equal(2, capped.Count);
+        Assert.Equal(pendingOld.NotificationId, capped[0].NotificationId);
+        Assert.Equal(retryingDue.NotificationId, capped[1].NotificationId);
+    }
+
+    [Fact]
+    public async Task GetByIdAsync_ReturnsRowOrNull()
+    {
+        var id = Guid.NewGuid().ToString();
+        _context.Notifications.Add(MakeNotification(id));
+        await _context.SaveChangesAsync();
+        _context.ChangeTracker.Clear();
+
+        Assert.NotNull(await _repository.GetByIdAsync(id));
+        Assert.Null(await _repository.GetByIdAsync("does-not-exist"));
+    }
+
+    [Fact]
+    public async Task UpdateAsync_PersistsStatusTransition()
+    {
+        var id = Guid.NewGuid().ToString();
+        _context.Notifications.Add(MakeNotification(id, NotificationStatus.Pending));
+        await _context.SaveChangesAsync();
+        _context.ChangeTracker.Clear();
+
+        var loaded = await _repository.GetByIdAsync(id);
+        loaded!.Status = NotificationStatus.Delivered;
+        loaded.DeliveredAt = new DateTimeOffset(2026, 5, 19, 9, 0, 0, TimeSpan.Zero);
+        await _repository.UpdateAsync(loaded);
+
+        _context.ChangeTracker.Clear();
+        var reloaded = await _context.Notifications.FindAsync(id);
+        Assert.Equal(NotificationStatus.Delivered, reloaded!.Status);
+        Assert.Equal(new DateTimeOffset(2026, 5, 19, 9, 0, 0, TimeSpan.Zero), reloaded.DeliveredAt);
+    }
+
+    [Fact]
+    public async Task QueryAsync_AppliesFilters_OrdersByCreatedAtDescending_AndPaginates()
+    {
+        var baseTime = new DateTimeOffset(2026, 5, 19, 8, 0, 0, TimeSpan.Zero);
+
+        // 5 matching rows for site-north / Ops List, plus noise.
+        for (var i = 0; i < 5; i++)
+        {
+            _context.Notifications.Add(MakeNotification(
+                Guid.NewGuid().ToString(), NotificationStatus.Pending,
+                createdAt: baseTime.AddMinutes(i),
+                subject: $"Tank Level {i}"));
+        }
+        _context.Notifications.Add(MakeNotification(Guid.NewGuid().ToString(),
+            sourceSiteId: "site-south", subject: "Other Site"));
+        await _context.SaveChangesAsync();
+        _context.ChangeTracker.Clear();
+
+        var filter = new NotificationOutboxFilter(SourceSiteId: "site-north", ListName: "Ops List");
+        var (rows, total) = await _repository.QueryAsync(filter, pageNumber: 1, pageSize: 3);
+
+        Assert.Equal(5, total);
+        Assert.Equal(3, rows.Count);
+        // Descending by CreatedAt: Tank Level 4, 3, 2.
+        Assert.Equal("Tank Level 4", rows[0].Subject);
+        Assert.Equal("Tank Level 3", rows[1].Subject);
+        Assert.Equal("Tank Level 2", rows[2].Subject);
+
+        var (page2, _) = await _repository.QueryAsync(filter, pageNumber: 2, pageSize: 3);
+        Assert.Equal(2, page2.Count);
+        Assert.Equal("Tank Level 1", page2[0].Subject);
+        Assert.Equal("Tank Level 0", page2[1].Subject);
+    }
+
+    [Fact]
+    public async Task QueryAsync_SubjectKeyword_UsesContains()
+    {
+        _context.Notifications.Add(MakeNotification(Guid.NewGuid().ToString(), subject: "Tank Level High"));
+        _context.Notifications.Add(MakeNotification(Guid.NewGuid().ToString(), subject: "Pump Failure"));
+        await _context.SaveChangesAsync();
+        _context.ChangeTracker.Clear();
+
+        var (rows, total) = await _repository.QueryAsync(
+            new NotificationOutboxFilter(SubjectKeyword: "Level"), pageNumber: 1, pageSize: 10);
+
+        Assert.Equal(1, total);
+        Assert.Equal("Tank Level High", rows[0].Subject);
+    }
+
+    [Fact]
+    public async Task QueryAsync_StuckOnly_ReturnsNonTerminalRowsOlderThanCutoff()
+    {
+        var cutoff = new DateTimeOffset(2026, 5, 19, 10, 0, 0, TimeSpan.Zero);
+
+        var stuckPending = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Pending,
+            createdAt: cutoff.AddHours(-2), subject: "Stuck Pending");
+        var stuckRetrying = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Retrying,
+            createdAt: cutoff.AddHours(-3), subject: "Stuck Retrying");
+        var freshPending = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Pending,
+            createdAt: cutoff.AddHours(1), subject: "Fresh");
+        var oldDelivered = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Delivered,
+            createdAt: cutoff.AddHours(-5), subject: "Old Delivered");
+
+        _context.Notifications.AddRange(stuckPending, stuckRetrying, freshPending, oldDelivered);
+        await _context.SaveChangesAsync();
+        _context.ChangeTracker.Clear();
+
+        var (rows, total) = await _repository.QueryAsync(
+            new NotificationOutboxFilter(StuckOnly: true, StuckCutoff: cutoff),
+            pageNumber: 1, pageSize: 10);
+
+        Assert.Equal(2, total);
+        Assert.Equal(
+            new[] { "Stuck Pending", "Stuck Retrying" },
+            rows.Select(r => r.Subject).OrderBy(s => s).ToArray());
+    }
+
+    [Fact]
+    public async Task QueryAsync_FromTo_FilterAgainstCreatedAt()
+    {
+        var baseTime = new DateTimeOffset(2026, 5, 19, 8, 0, 0, TimeSpan.Zero);
+        for (var i = 0; i < 5; i++)
+        {
+            _context.Notifications.Add(MakeNotification(Guid.NewGuid().ToString(),
+                createdAt: baseTime.AddHours(i), subject: $"Row {i}"));
+        }
+        await _context.SaveChangesAsync();
+        _context.ChangeTracker.Clear();
+
+        var (rows, total) = await _repository.QueryAsync(
+            new NotificationOutboxFilter(From: baseTime.AddHours(1), To: baseTime.AddHours(3)),
+            pageNumber: 1, pageSize: 10);
+
+        Assert.Equal(3, total);
+        Assert.Equal(
+            new[] { "Row 1", "Row 2", "Row 3" },
+            rows.Select(r => r.Subject).OrderBy(s => s).ToArray());
+    }
+
+    [Fact]
+    public async Task QueryAsync_StatusAndTypeFilters()
+    {
+        _context.Notifications.Add(MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Parked,
+            subject: "Parked Row"));
+        _context.Notifications.Add(MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Pending,
+            subject: "Pending Row"));
+        await _context.SaveChangesAsync();
+        _context.ChangeTracker.Clear();
+
+        var (rows, total) = await _repository.QueryAsync(
+            new NotificationOutboxFilter(Status: NotificationStatus.Parked, Type: NotificationType.Email),
+            pageNumber: 1, pageSize: 10);
+
+        Assert.Equal(1, total);
+        Assert.Equal("Parked Row", rows[0].Subject);
+    }
+
+    [Fact]
+    public async Task DeleteTerminalOlderThanAsync_DeletesTerminalRowsOlderThanCutoff_LeavesOthers()
+    {
+        var cutoff = new DateTimeOffset(2026, 5, 19, 10, 0, 0, TimeSpan.Zero);
+
+        var oldDelivered = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Delivered,
+            createdAt: cutoff.AddHours(-1));
+        var oldParked = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Parked,
+            createdAt: cutoff.AddHours(-2));
+        var oldDiscarded = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Discarded,
+            createdAt: cutoff.AddHours(-3));
+        var recentDelivered = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Delivered,
+            createdAt: cutoff.AddHours(1));
+        var oldPending = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Pending,
+            createdAt: cutoff.AddHours(-4));
+
+        _context.Notifications.AddRange(oldDelivered, oldParked, oldDiscarded, recentDelivered, oldPending);
+        await _context.SaveChangesAsync();
+        _context.ChangeTracker.Clear();
+
+        var deleted = await _repository.DeleteTerminalOlderThanAsync(cutoff);
+
+        Assert.Equal(3, deleted);
+        var remaining = await _context.Notifications.Select(n => n.NotificationId).ToListAsync();
+        Assert.Equal(2, remaining.Count);
+        Assert.Contains(recentDelivered.NotificationId, remaining);
+        Assert.Contains(oldPending.NotificationId, remaining);
+    }
+
+    [Fact]
+    public async Task ComputeKpisAsync_ComputesSnapshotFields()
+    {
+        var now = DateTimeOffset.UtcNow;
+        var stuckCutoff = now.AddMinutes(-30);
+        var deliveredSince = now.AddHours(-1);
+
+        var oldestPending = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Pending,
+            createdAt: now.AddHours(-2));   // stuck
+        var freshPending = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Pending,
+            createdAt: now.AddMinutes(-5)); // not stuck
+        var stuckRetrying = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Retrying,
+            createdAt: now.AddMinutes(-45)); // stuck
+        var parked = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Parked,
+            createdAt: now.AddHours(-3));
+        var recentlyDelivered = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Delivered,
+            createdAt: now.AddHours(-4), deliveredAt: now.AddMinutes(-10));
+        var oldDelivered = MakeNotification(Guid.NewGuid().ToString(), NotificationStatus.Delivered,
+            createdAt: now.AddHours(-5), deliveredAt: now.AddHours(-2));
+
+        _context.Notifications.AddRange(oldestPending, freshPending, stuckRetrying, parked,
+            recentlyDelivered, oldDelivered);
+        await _context.SaveChangesAsync();
+        _context.ChangeTracker.Clear();
+
+        var kpis = await _repository.ComputeKpisAsync(stuckCutoff, deliveredSince);
+
+        Assert.Equal(3, kpis.QueueDepth);             // 2 pending + 1 retrying
+        Assert.Equal(2, kpis.StuckCount);             // oldestPending + stuckRetrying
+        Assert.Equal(1, kpis.ParkedCount);            // parked
+        Assert.Equal(1, kpis.DeliveredLastInterval);  // recentlyDelivered only
+        Assert.NotNull(kpis.OldestPendingAge);
+        // Oldest non-terminal row is oldestPending (created ~2h ago).
+        Assert.True(kpis.OldestPendingAge!.Value >= TimeSpan.FromMinutes(115));
+    }
+
+    [Fact]
+    public async Task ComputeKpisAsync_NoNonTerminalRows_OldestPendingAgeIsNull()
+    {
+        var now = DateTimeOffset.UtcNow;
+        _context.Notifications.Add(MakeNotification(Guid.NewGuid().ToString(),
+            NotificationStatus.Delivered, createdAt: now.AddHours(-1), deliveredAt: now.AddMinutes(-5)));
+        await _context.SaveChangesAsync();
+        _context.ChangeTracker.Clear();
+
+        var kpis = await _repository.ComputeKpisAsync(now.AddMinutes(-30), now.AddHours(-1));
+
+        Assert.Equal(0, kpis.QueueDepth);
+        Assert.Equal(0, kpis.StuckCount);
+        Assert.Null(kpis.OldestPendingAge);
+    }
+
+    [Fact]
+    public void Constructor_NullContext_Throws()
+    {
+        Assert.Throws<ArgumentNullException>(() => new NotificationOutboxRepository(null!));
+    }
+}
+
 public class SiteRepositoryTests : IDisposable
 {
     private readonly ScadaLinkDbContext _context;