feat(deploy): wire periodic PendingDeployment purge + SQL Server same-id re-stage test

Notify-and-fetch follow-ups:

- PendingDeploymentPurgeActor: a central cluster singleton (not
  readiness-gated, best-effort) that sweeps expired PendingDeployment
  staging rows on CommunicationOptions.PendingDeploymentPurgeInterval
  (default 1h). Modeled on the kpi-history-recorder pattern: self-scheduling
  timer, per-tick DI scope -> IDeploymentManagerRepository, continue-on-error.
  Wired in AkkaHostedService.RegisterCentralActors (manager + proxy + drain);
  resolves the deferred TODO in DeploymentService. Correctness never depends
  on it (supersession bounds rows to <=1/instance; the fetch endpoint enforces
  the TTL), so it is deliberately absent from RequiredSingletonsHealthCheck.

- SQL Server integration test for StagePendingIfAbsentAsync re-staging an
  instance's OWN DeploymentId over an expired row against the real UNIQUE
  index on DeploymentId — confirms EF orders DELETE before INSERT in one
  SaveChanges (SQLite's constraint timing differs from SQL Server's). Plus
  a same-instance supersession variant on real SQL Server.

Tests: 2 TestKit actor tests + 2 SQL Server integration tests (both ran
green against the infra MSSQL container); 235 Communication + 15
PendingDeployment tests pass; Host builds 0 warnings.

src/ZB.MOM.WW.ScadaBridge.DeploymentManager/DeploymentService.cs

@@ -285,10 +285,9 @@ public class DeploymentService
             // standby node to fetch; deleting now would 404 that in-flight
             // standby fetch and break failover. Supersession bounds pending rows
             // to ≤1 per instance and the fetch endpoint enforces the TTL, so
-            // leaving rows for TTL purge is safe.
-            // TODO(notify-and-fetch): wire PurgeExpiredPendingDeploymentsAsync
-            // into a central maintenance cadence (none exists in DeploymentManager
-            // today; deferred — supersession + endpoint TTL keep this safe).
+            // leaving rows for TTL purge is safe. Expired rows are swept by the
+            // central PendingDeploymentPurgeActor singleton on its
+            // CommunicationOptions.PendingDeploymentPurgeInterval cadence.
             var response = await _communicationService.RefreshDeploymentAsync(siteId, command, cancellationToken);
 
             // WP-1: Update status based on site response.