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.Communication/CommunicationOptions.cs

@@ -71,4 +71,13 @@ public class CommunicationOptions
     /// comfortably cover both site nodes' fetches within one deploy window.
     /// </summary>
     public TimeSpan PendingDeploymentTtl { get; set; } = TimeSpan.FromMinutes(5);
+
+    /// <summary>
+    /// How often the central <c>PendingDeploymentPurgeActor</c> singleton reclaims
+    /// expired (TTL-elapsed) PendingDeployment staging rows. Best-effort hygiene only:
+    /// supersession bounds pending rows to ≤1 per instance and the config-fetch endpoint
+    /// already enforces the TTL, so this purge merely sweeps rows left behind by instances
+    /// that are deployed once and never re-deployed. Default 1 hour ≫ <see cref="PendingDeploymentTtl"/>.
+    /// </summary>
+    public TimeSpan PendingDeploymentPurgeInterval { get; set; } = TimeSpan.FromHours(1);
 }