Task 11: cluster protocol — subscribe(), ping/pong health, 9 message types

- amqp.py: subscribe() creates exclusive queues bound to routing keys,
  _rebind_subscriptions() recreates them after reconnect
- cluster.py: CLUSTER_NODES, CLUSTER_EVENTS (bounded 1000),
  CLUSTER_COORDINATOR with auto-promotion, all 6 handlers
  (register, deregister, pong, event, coordinator_query),
  ping_node() with 5s timeout + auto-deregister on failure
- routers/cluster.py: GET /api/cluster
- Wired into app.py lifespan after amqp_connect()
- 13 tests covering all handlers, boundaries, and API shape
- No passive heartbeats — workers assumed present until ping
  timeout at work-routing time

routers/cluster.py

@@ -0,0 +1,23 @@
+"""JarvisChat routers - Cluster status API."""
+from fastapi import APIRouter
+
+import cluster
+
+router = APIRouter()
+
+
+@router.get("/api/cluster")
+async def cluster_status():
+    return {
+        "nodes": {name: _strip_internal(node) for name, node in cluster.CLUSTER_NODES.items()},
+        "node_count": len(cluster.CLUSTER_NODES),
+        "coordinator": cluster.CLUSTER_COORDINATOR,
+        "events": list(cluster.CLUSTER_EVENTS),
+    }
+
+
+def _strip_internal(node: dict) -> dict:
+    return {k: v for k, v in node.items() if k in {
+        "name", "type", "status", "capabilities", "active_model", "load",
+        "registered_at", "last_seen",
+    }}