feat(notification-outbox): per-site KPI aggregation in the repository

src/ScadaLink.ConfigurationDatabase/Repositories/NotificationOutboxRepository.cs

@@ -174,6 +174,68 @@ public class NotificationOutboxRepository : INotificationOutboxRepository
             OldestPendingAge: oldestPendingAge);
     }
 
+    public async Task<IReadOnlyList<SiteNotificationKpiSnapshot>> ComputePerSiteKpisAsync(
+        DateTimeOffset stuckCutoff, DateTimeOffset deliveredSince, CancellationToken cancellationToken = default)
+    {
+        var now = DateTimeOffset.UtcNow;
+
+        var queueDepth = await CountBySiteAsync(
+            n => n.Status == NotificationStatus.Pending || n.Status == NotificationStatus.Retrying,
+            cancellationToken);
+
+        var stuck = await CountBySiteAsync(
+            n => (n.Status == NotificationStatus.Pending || n.Status == NotificationStatus.Retrying)
+                && n.CreatedAt < stuckCutoff,
+            cancellationToken);
+
+        var parked = await CountBySiteAsync(
+            n => n.Status == NotificationStatus.Parked, cancellationToken);
+
+        var delivered = await CountBySiteAsync(
+            n => n.Status == NotificationStatus.Delivered
+                && n.DeliveredAt != null && n.DeliveredAt >= deliveredSince,
+            cancellationToken);
+
+        // Oldest non-terminal CreatedAt per site. A SQL Min over the DateTimeOffset
+        // converter is awkward (see ComputeKpisAsync), so project the non-terminal
+        // (site, created) pairs — the live queue, which stays bounded — and reduce
+        // in memory.
+        var oldest = (await _context.Notifications
+                .Where(n => n.Status == NotificationStatus.Pending
+                    || n.Status == NotificationStatus.Retrying)
+                .Select(n => new { n.SourceSiteId, n.CreatedAt })
+                .ToListAsync(cancellationToken))
+            .GroupBy(x => x.SourceSiteId)
+            .ToDictionary(g => g.Key, g => g.Min(x => x.CreatedAt));
+
+        var siteIds = queueDepth.Keys
+            .Concat(stuck.Keys).Concat(parked.Keys).Concat(delivered.Keys)
+            .Distinct()
+            .OrderBy(s => s, StringComparer.Ordinal);
+
+        return siteIds.Select(site => new SiteNotificationKpiSnapshot(
+            SourceSiteId: site,
+            QueueDepth: queueDepth.GetValueOrDefault(site),
+            StuckCount: stuck.GetValueOrDefault(site),
+            ParkedCount: parked.GetValueOrDefault(site),
+            DeliveredLastInterval: delivered.GetValueOrDefault(site),
+            OldestPendingAge: oldest.TryGetValue(site, out var createdAt)
+                ? now - createdAt
+                : null)).ToList();
+    }
+
+    /// <summary>Counts notification rows matching <paramref name="predicate"/>, grouped by source site.</summary>
+    private async Task<Dictionary<string, int>> CountBySiteAsync(
+        System.Linq.Expressions.Expression<Func<Notification, bool>> predicate,
+        CancellationToken cancellationToken)
+    {
+        return await _context.Notifications
+            .Where(predicate)
+            .GroupBy(n => n.SourceSiteId)
+            .Select(g => new { Site = g.Key, Count = g.Count() })
+            .ToDictionaryAsync(x => x.Site, x => x.Count, cancellationToken);
+    }
+
     public async Task<int> SaveChangesAsync(CancellationToken cancellationToken = default)
         => await _context.SaveChangesAsync(cancellationToken);
 }