feat: add router health metrics to node_cache and node-guardian

- Add migration 042_node_cache_router_metrics.sql
- Node guardian now collects router health and sends in heartbeat
- City-service uses cached router_healthy from node_cache
- This allows NODE2 router status to be displayed correctly

scripts/node-guardian-loop.py

@@ -255,6 +255,20 @@ class NodeGuardian:
                 
         except Exception as e:
             logger.warning(f"Swapper metrics collection failed: {e}")
+        
+        # Collect Router Metrics using node-specific URL
+        metrics["router_healthy"] = False
+        metrics["router_version"] = None
+        try:
+            r = await self.client.get(f"{self.router_url}/health", timeout=3.0)
+            if r.status_code == 200:
+                data = r.json()
+                status = data.get("status", "").lower()
+                metrics["router_healthy"] = status in ("healthy", "ok")
+                metrics["router_version"] = data.get("version")
+                logger.debug(f"🔀 Router metrics: healthy={metrics['router_healthy']}, version={metrics['router_version']}")
+        except Exception as e:
+            logger.debug(f"Router health check failed: {e}")
             
         return metrics