feat(localdb): enable replication on the site-a rig pair

Task 11. site-a-a is the initiator (dials scadabridge-site-a-b:8083, the
existing site gRPC h2c listener - no new port); site-a-b is passive. Both carry
the SAME dev ApiKey, which is load-bearing: LocalDbSyncAuthInterceptor is
fail-closed, so a mismatch does not degrade to unauthenticated replication, it
rejects every stream and the pair silently stops converging.

site-b and site-c stay unreplicated on purpose, so the default-OFF posture is
proven side-by-side on one rig rather than asserted.

Plain dev key matches the rig's existing posture; production uses a
${secret:...} reference through the pre-host expander. Noted in both files.

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

docker/site-a-node-a/appsettings.Site.json

@@ -64,6 +64,23 @@
   // outside the volume and were lost on every recreate.
   // Replication is opt-in and configured separately; absent = local-only.
   "LocalDb": {
-    "Path": "/app/data/site-localdb.db"
+    "Path": "/app/data/site-localdb.db",
+    // Site-a is the rig's REPLICATED pair; site-b and site-c deliberately stay
+    // unreplicated so the default-OFF posture is proven side-by-side on one rig.
+    //
+    // This node is the initiator: it dials the peer. Replication is still
+    // bidirectional - the passive node's writes flow back over the same stream -
+    // so only one side needs PeerAddress. Port 8083 is the existing site gRPC
+    // listener (h2c); the sync endpoint shares it, no new port.
+    //
+    // The ApiKey must be IDENTICAL on both nodes: the host's
+    // LocalDbSyncAuthInterceptor is fail-closed, so a mismatch (or a missing key)
+    // rejects every sync stream. A plain dev key here matches the rig's existing
+    // posture; PRODUCTION uses a "${secret:...}" reference resolved by the
+    // pre-host secret expander.
+    "Replication": {
+      "PeerAddress": "http://scadabridge-site-a-b:8083",
+      "ApiKey": "dev-site-a-localdb-sync-key"
+    }
   }
 }

docker/site-a-node-b/appsettings.Site.json

@@ -64,6 +64,16 @@
   // outside the volume and were lost on every recreate.
   // Replication is opt-in and configured separately; absent = local-only.
   "LocalDb": {
-    "Path": "/app/data/site-localdb.db"
+    "Path": "/app/data/site-localdb.db",
+    // The PASSIVE half of site-a's replicated pair: no PeerAddress, so this node's
+    // sync initiator starts and idles while node-a dials in. Its own writes still
+    // reach node-a - the stream is bidirectional.
+    //
+    // The key MUST match node-a's exactly. LocalDbSyncAuthInterceptor is
+    // fail-closed, so a typo here does not degrade to unauthenticated replication;
+    // it rejects every stream and the pair silently stops converging.
+    "Replication": {
+      "ApiKey": "dev-site-a-localdb-sync-key"
+    }
   }
 }