feat: add caching for peer proxy requests

- Cache successful GET responses from peers (5 min TTL)
- Return cached data when peer is unreachable
- Add _cached, _cached_at, _cache_age_seconds flags
- Add /api/node/<id>/health proxy endpoint

Enables master dashboard to show peer data even when
peers are temporarily unreachable (e.g., mobile devices,
VPN disconnections).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/rf_mapper/web/app.py

@@ -1679,12 +1679,21 @@ def create_app(config: Config | None = None) -> Flask:
 
     # ==================== Multi-Node Master Dashboard Proxy API ====================
 
+    # Cache for peer responses (scanner_id -> path -> {data, timestamp})
+    _peer_cache: dict[str, dict[str, dict]] = {}
+    _peer_cache_ttl = 300  # 5 minutes cache TTL
+
     def proxy_peer_request(scanner_id: str, path: str, method: str = "GET", **kwargs):
-        """Proxy a request to a peer node.
+        """Proxy a request to a peer node with caching.
 
         Returns:
             - Response tuple (jsonify(...), status_code) on error or peer response
             - None if scanner_id matches local scanner (caller should use local handler)
+
+        Caching behavior:
+            - GET requests are cached on success
+            - On peer unreachable, returns cached data if available (with _cached flag)
+            - POST requests are not cached but will return cached GET data on failure
         """
         local_id = app.config.get("SCANNER_IDENTITY", {}).get("id")
         if scanner_id == local_id:
@@ -1695,10 +1704,13 @@ def create_app(config: Config | None = None) -> Flask:
         if not peer or not peer.get("url"):
             return jsonify({"error": "Peer not found"}), 404
 
+        cache_key = f"{scanner_id}:{path}"
+
         try:
             url = f"{peer['url']}{path}"
-            resp = requests.request(method, url, timeout=15, **kwargs)
+            resp = requests.request(method, url, timeout=10, **kwargs)
             data = resp.json()
+
             # Enrich response with source scanner info
             data["_source_scanner"] = scanner_id
             data["_source_position"] = {
@@ -1706,9 +1718,39 @@ def create_app(config: Config | None = None) -> Flask:
                 "lon": peer.get("longitude"),
                 "floor": peer.get("floor")
             }
+            data["_cached"] = False
+
+            # Cache successful GET responses
+            if method == "GET":
+                if scanner_id not in _peer_cache:
+                    _peer_cache[scanner_id] = {}
+                _peer_cache[scanner_id][path] = {
+                    "data": data,
+                    "timestamp": datetime.now()
+                }
+
             return jsonify(data)
+
         except requests.RequestException as e:
-            return jsonify({"error": f"Peer unreachable: {e}"}), 503
+            # Try to return cached data
+            if scanner_id in _peer_cache and path in _peer_cache[scanner_id]:
+                cached = _peer_cache[scanner_id][path]
+                cache_age = (datetime.now() - cached["timestamp"]).total_seconds()
+
+                # Return cached data if within TTL (or always for display purposes)
+                cached_data = cached["data"].copy()
+                cached_data["_cached"] = True
+                cached_data["_cached_at"] = cached["timestamp"].isoformat()
+                cached_data["_cache_age_seconds"] = int(cache_age)
+                cached_data["_peer_error"] = str(e)
+
+                return jsonify(cached_data)
+
+            return jsonify({
+                "error": f"Peer unreachable: {e}",
+                "_source_scanner": scanner_id,
+                "_cached": False
+            }), 503
 
     @app.route("/api/node/<scanner_id>/latest")
     def api_node_latest(scanner_id: str):
@@ -1742,6 +1784,14 @@ def create_app(config: Config | None = None) -> Flask:
             return api_trilaterated_positions()
         return result
 
+    @app.route("/api/node/<scanner_id>/health")
+    def api_node_health(scanner_id: str):
+        """Proxy: Get health status from a peer node."""
+        result = proxy_peer_request(scanner_id, "/api/health")
+        if result is None:
+            return api_health()
+        return result
+
     return app