feat(localdb): periodic oplog maintenance service (caps enforcement + tombstone pruning, both roles)

Claude-Session: https://claude.ai/code/session_01BL2Vu1ESDQ9SCN4gVKkdts

ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/DependencyInjection/ReplicationServiceCollectionExtensions.cs

@@ -16,7 +16,9 @@ public static class ReplicationServiceCollectionExtensions
     /// configuration section, registers the passive <see cref="LocalDbSyncService"/> as a singleton
     /// (its single-stream guard must span calls), and registers the initiating
     /// <see cref="SyncBackgroundService"/> as a hosted service — on a passive node (no
-    /// <see cref="ReplicationOptions.PeerAddress"/>) it starts and immediately idles. Idempotent.
+    /// <see cref="ReplicationOptions.PeerAddress"/>) it starts and immediately idles. The periodic
+    /// <see cref="MaintenanceBackgroundService"/> (oplog caps enforcement + tombstone pruning) is
+    /// also always registered and runs on BOTH roles — passive nodes need it too. Idempotent.
     /// Requires <see cref="LocalDbServiceCollectionExtensions.AddZbLocalDb"/> to have been called
     /// for the <see cref="ILocalDb"/> the adapters depend on.
     /// </summary>
@@ -59,6 +61,11 @@ public static class ReplicationServiceCollectionExtensions
 
         services.AddSingleton<LocalDbSyncService>();
         services.AddHostedService<SyncBackgroundService>();
+        // Factory registration because the ctor takes the internal shared OplogStore singleton.
+        services.AddHostedService(sp => new MaintenanceBackgroundService(
+            sp.GetRequiredService<OplogStore>(),
+            sp.GetRequiredService<IOptions<ReplicationOptions>>(),
+            sp.GetRequiredService<Microsoft.Extensions.Logging.ILoggerFactory>()));
 
         return services;
     }