feat(configdb): AuditLogRepository (EF) + DI registration, append-only with M6-deferred SwitchOutPartition (#23)

EF Core implementation of IAuditLogRepository:
- InsertIfNotExistsAsync: single IF NOT EXISTS ... INSERT via
  ExecuteSqlInterpolatedAsync, bypasses the change tracker. Enum
  values converted to string in C# (columns are varchar(32) via
  HasConversion<string>).
- QueryAsync: AsNoTracking, predicate-per-non-null-filter, keyset
  paging on (OccurredAtUtc DESC, EventId DESC) — EF Core 10
  translates Guid.CompareTo to a uniqueidentifier < comparison
  natively (verified against MSSQL 2022).
- SwitchOutPartitionAsync: throws NotSupportedException naming M6;
  the non-aligned UX_AuditLog_EventId unique index blocks
  ALTER TABLE SWITCH PARTITION until the drop-and-rebuild dance
  ships with the purge actor.

DI: AddScoped<IAuditLogRepository, AuditLogRepository>() added after
the NotificationOutboxRepository registration; existing DI smoke test
extended with an IAuditLogRepository assertion.

Integration tests (8 new) use the Bundle C MsSqlMigrationFixture and
scope by a per-test SourceSiteId guid so they neither collide nor
require cleanup.

Bundle D of the Audit Log #23 M1 Foundation plan.

src/ScadaLink.ConfigurationDatabase/ServiceCollectionExtensions.cs

@@ -46,6 +46,7 @@ public static class ServiceCollectionExtensions
         services.AddScoped<IExternalSystemRepository, ExternalSystemRepository>();
         services.AddScoped<INotificationRepository, NotificationRepository>();
         services.AddScoped<INotificationOutboxRepository, NotificationOutboxRepository>();
+        services.AddScoped<IAuditLogRepository, AuditLogRepository>();
         services.AddScoped<IInboundApiRepository, InboundApiRepository>();
         services.AddScoped<IAuditService, AuditService>();
         services.AddScoped<IInstanceLocator, InstanceLocator>();