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.

tests/ScadaLink.ConfigurationDatabase.Tests/ServiceCollectionExtensionsTests.cs

@@ -18,6 +18,7 @@ public class ServiceCollectionExtensionsTests
         Assert.Contains(services, d => d.ServiceType == typeof(ITemplateEngineRepository));
         Assert.Contains(services, d => d.ServiceType == typeof(IAuditService));
         Assert.Contains(services, d => d.ServiceType == typeof(IInstanceLocator));
+        Assert.Contains(services, d => d.ServiceType == typeof(IAuditLogRepository));
     }
 
     // The no-arg overload is [Obsolete(error: true)], so it cannot be referenced directly