fix(configdb): replace SwitchOutPartitionAsync stub with drop-and-rebuild dance (#23 M6)

Replaces M1's NotSupportedException stub with the production drop-DROP-INDEX
→ CREATE-staging → SWITCH PARTITION → DROP-staging → CREATE-INDEX dance
documented in alog.md §4. UX_AuditLog_EventId is intentionally non-aligned
with ps_AuditLog_Month so single-column EventId uniqueness can be enforced
cheaply for InsertIfNotExistsAsync; SQL Server rejects ALTER TABLE SWITCH
while a non-aligned unique index is present, so the implementation drops
it, switches the partition data into a GUID-suffixed staging table on
[PRIMARY], drops staging (discarding the rows), and rebuilds the unique
index — all inside an explicit transaction with a CATCH that guarantees
the unique index is rebuilt regardless of failure point.

Also adds GetPartitionBoundariesOlderThanAsync to IAuditLogRepository: a
CROSS APPLY over sys.partition_range_values + per-partition MAX(OccurredAtUtc)
to enumerate retention-eligible months for the M6 purge actor (next commit).

Tests verify:
* Old partition's rows are removed; other months untouched
* UX_AuditLog_EventId is rebuilt after a successful switch
* InsertIfNotExistsAsync's first-write-wins idempotency still holds after switch
* On engineered SWITCH failure (inbound FK from a probe table), SqlException
  propagates AND UX_AuditLog_EventId is still present (CATCH branch ran)
* GetPartitionBoundariesOlderThanAsync returns only boundaries whose partition's
  MAX(OccurredAtUtc) is strictly older than the threshold; empty partitions
  excluded

tests/ScadaLink.AuditLog.Tests/Central/AuditLogIngestActorTests.cs

@@ -216,5 +216,9 @@ public class AuditLogIngestActorTests : TestKit, IClassFixture<MsSqlMigrationFix
 
         public Task SwitchOutPartitionAsync(DateTime monthBoundary, CancellationToken ct = default) =>
             _inner.SwitchOutPartitionAsync(monthBoundary, ct);
+
+        public Task<IReadOnlyList<DateTime>> GetPartitionBoundariesOlderThanAsync(
+            DateTime threshold, CancellationToken ct = default) =>
+            _inner.GetPartitionBoundariesOlderThanAsync(threshold, ct);
     }
 }

tests/ScadaLink.AuditLog.Tests/Central/SiteAuditReconciliationActorTests.cs

@@ -89,6 +89,10 @@ public class SiteAuditReconciliationActorTests : TestKit, IClassFixture<MsSqlMig
 
         public Task SwitchOutPartitionAsync(DateTime monthBoundary, CancellationToken ct = default) =>
             Task.CompletedTask;
+
+        public Task<IReadOnlyList<DateTime>> GetPartitionBoundariesOlderThanAsync(
+            DateTime threshold, CancellationToken ct = default) =>
+            Task.FromResult<IReadOnlyList<DateTime>>(Array.Empty<DateTime>());
     }
 
     /// <summary>