feat(configdb): composite PK + UX_AuditLog_EventId on AuditEvent mapping (#23)

Bundle C migration aligns AuditLog to ps_AuditLog_Month(OccurredAtUtc).
A partitioned table's clustered key must include the partition column, so
the PK becomes the composite {EventId, OccurredAtUtc} — a divergence from
Bundle B's single-column PK that needs reconciling in the EF mapping
before the migration is generated.

EventId remains the idempotency key for AuditLogRepository.InsertIfNotExistsAsync
(M1-T8), so a dedicated unique index UX_AuditLog_EventId preserves the
single-column uniqueness constraint.

Tests updated:
- Configure_MapsToAuditLogTable_WithCompositePrimaryKey (replaces the
  WithEventIdAsPrimaryKey assertion) verifies {EventId, OccurredAtUtc}.
- Configure_DeclaresUniqueIndex_OnEventIdAlone_ForIdempotencyLookups
  asserts the new UX_AuditLog_EventId is unique and on EventId alone.
- Configure_ExpectedIndexes_WithCorrectNames now expects six index names
  (the original five plus UX_AuditLog_EventId).

src/ScadaLink.ConfigurationDatabase/Configurations/AuditLogEntityTypeConfiguration.cs

@@ -15,7 +15,15 @@ public class AuditLogEntityTypeConfiguration : IEntityTypeConfiguration<AuditEve
     {
         builder.ToTable("AuditLog");
 
-        builder.HasKey(e => e.EventId);
+        // Composite PK includes OccurredAtUtc — required by the monthly partition scheme
+        // (ps_AuditLog_Month) so the clustered key is partition-aligned. EventId still
+        // needs to be globally unique for InsertIfNotExistsAsync idempotency, so a
+        // separate unique index is declared on EventId alone.
+        builder.HasKey(e => new { e.EventId, e.OccurredAtUtc });
+
+        builder.HasIndex(e => e.EventId)
+            .IsUnique()
+            .HasDatabaseName("UX_AuditLog_EventId");
 
         // Enum-as-string columns: bounded varchar(32) ASCII.
         builder.Property(e => e.Channel)