feat(configdb): map SiteCall to SiteCalls table (#22, #23 M3)

Bundle B1 of Audit Log #23 M3: introduces the SiteCall entity + EF mapping
for the central SiteCalls operational-state table. One row per
TrackedOperationId, mirrored from sites via best-effort telemetry then
periodic reconciliation; eventually-consistent mirror, not a dispatcher.

- src/ScadaLink.Commons/Entities/Audit/SiteCall.cs: append-once record
  with required TrackedOperationId/Channel/Target/SourceSite/Status,
  monotonic status update at the repo layer.
- src/ScadaLink.ConfigurationDatabase/Configurations/SiteCallEntityTypeConfiguration.cs:
  table SiteCalls, PK on TrackedOperationId (stored as varchar(36) via
  value conversion through the canonical 'D'-format GUID string —
  matches the wire shape used by gRPC + SQLite columns), two named
  indexes (IX_SiteCalls_Source_Created, IX_SiteCalls_Status_Updated).
- ScadaLinkDbContext: DbSet<SiteCall> SiteCalls in the existing Audit
  section, after AuditLogs.
- Tests in tests/ScadaLink.ConfigurationDatabase.Tests/Configurations/:
  table name, PK, value-conversion shape, index presence + ordering.

tests/ScadaLink.ConfigurationDatabase.Tests/Configurations/SiteCallEntityTypeConfigurationTests.cs

@@ -0,0 +1,109 @@
+using Microsoft.EntityFrameworkCore;
+using ScadaLink.Commons.Entities.Audit;
+using ScadaLink.ConfigurationDatabase;
+using ScadaLink.ConfigurationDatabase.Configurations;
+
+namespace ScadaLink.ConfigurationDatabase.Tests.Configurations;
+
+/// <summary>
+/// Schema-level tests for <see cref="SiteCallEntityTypeConfiguration"/> (#22 / #23 M3 Bundle B).
+/// Verifies the <see cref="SiteCall"/> record maps to the <c>SiteCalls</c> table with the
+/// expected primary key, value conversion on <c>TrackedOperationId</c>, and the two named
+/// indexes that back the "calls from this site" and "calls in this status" Central UI queries.
+/// Mirrors the AuditLog Bundle B test pattern — inspects EF model metadata via the existing
+/// in-memory SQLite test context, no database round-trips required.
+/// </summary>
+public class SiteCallEntityTypeConfigurationTests : IDisposable
+{
+    private readonly ScadaLinkDbContext _context;
+
+    public SiteCallEntityTypeConfigurationTests()
+    {
+        _context = SqliteTestHelper.CreateInMemoryContext();
+    }
+
+    public void Dispose()
+    {
+        _context.Database.CloseConnection();
+        _context.Dispose();
+    }
+
+    [Fact]
+    public void Configure_MapsToSiteCallsTable()
+    {
+        var entity = _context.Model.FindEntityType(typeof(SiteCall));
+
+        Assert.NotNull(entity);
+        Assert.Equal("SiteCalls", entity!.GetTableName());
+    }
+
+    [Fact]
+    public void Configure_PrimaryKey_TrackedOperationId()
+    {
+        var entity = _context.Model.FindEntityType(typeof(SiteCall));
+        Assert.NotNull(entity);
+
+        var pk = entity!.FindPrimaryKey();
+        Assert.NotNull(pk);
+
+        var pkPropertyNames = pk!.Properties.Select(p => p.Name).ToArray();
+        Assert.Equal(new[] { nameof(SiteCall.TrackedOperationId) }, pkPropertyNames);
+    }
+
+    [Fact]
+    public void Configure_HasIndexes_NamedAndOrdered()
+    {
+        var entity = _context.Model.FindEntityType(typeof(SiteCall));
+        Assert.NotNull(entity);
+
+        var indexes = entity!.GetIndexes().ToList();
+
+        // IX_SiteCalls_Source_Created: (SourceSite ASC, CreatedAtUtc DESC).
+        var sourceCreated = indexes.SingleOrDefault(i => i.GetDatabaseName() == "IX_SiteCalls_Source_Created");
+        Assert.NotNull(sourceCreated);
+        var sourceCreatedProps = sourceCreated!.Properties.Select(p => p.Name).ToArray();
+        Assert.Equal(new[] { nameof(SiteCall.SourceSite), nameof(SiteCall.CreatedAtUtc) }, sourceCreatedProps);
+
+        // IX_SiteCalls_Status_Updated: (Status ASC, UpdatedAtUtc DESC).
+        var statusUpdated = indexes.SingleOrDefault(i => i.GetDatabaseName() == "IX_SiteCalls_Status_Updated");
+        Assert.NotNull(statusUpdated);
+        var statusUpdatedProps = statusUpdated!.Properties.Select(p => p.Name).ToArray();
+        Assert.Equal(new[] { nameof(SiteCall.Status), nameof(SiteCall.UpdatedAtUtc) }, statusUpdatedProps);
+    }
+
+    [Fact]
+    public void Configure_TrackedOperationId_ConvertedToString_Length36()
+    {
+        var entity = _context.Model.FindEntityType(typeof(SiteCall));
+        Assert.NotNull(entity);
+
+        var property = entity!.FindProperty(nameof(SiteCall.TrackedOperationId));
+        Assert.NotNull(property);
+
+        // Stored as varchar(36) (TrackedOperationId.ToString("D") is always 36 chars).
+        // The value-conversion target type is exposed via GetProviderClrType when set, or
+        // discovered indirectly through the configured converter; either way the on-wire
+        // CLR type is string.
+        var providerClrType = property!.GetProviderClrType() ?? property.GetValueConverter()?.ProviderClrType;
+        Assert.Equal(typeof(string), providerClrType);
+        Assert.Equal(36, property.GetMaxLength());
+        Assert.False(property.IsUnicode() ?? true);
+    }
+
+    [Theory]
+    [InlineData(nameof(SiteCall.Channel), 32)]
+    [InlineData(nameof(SiteCall.SourceSite), 64)]
+    [InlineData(nameof(SiteCall.Status), 32)]
+    [InlineData(nameof(SiteCall.Target), 256)]
+    public void Configure_AsciiBoundedColumns(string propertyName, int expectedMaxLength)
+    {
+        var entity = _context.Model.FindEntityType(typeof(SiteCall));
+        Assert.NotNull(entity);
+
+        var property = entity!.FindProperty(propertyName);
+        Assert.NotNull(property);
+
+        Assert.Equal(expectedMaxLength, property!.GetMaxLength());
+        Assert.False(property.IsUnicode() ?? true);
+    }
+}