Fix 7 bugs: security, memory leak, stale state, error handling

- WebSocket: verify task ownership before allowing subscribe (security)
- POI routes: replace assert with HTTPException for production safety
- cancel_task: return HTTP 404 instead of 200 for missing tasks
- routing_config: add descriptive ValueError for invalid env vars
- POIManager: show error feedback instead of silently swallowing failures
- VisualizationCard: reset POI/travel mode state on metric switch
- Map: clean up heatmap layers/sources on unmount to prevent memory leak
- Update test to expect 404 from cancel_task ownership check

api/ws_routes.py

@@ -137,6 +137,10 @@ async def ws_task_progress(websocket: WebSocket) -> None:
             if msg_type == "subscribe":
                 new_task_id = msg.get("task_id")
                 if new_task_id:
+                    # Verify task belongs to the authenticated user
+                    user_tasks = task_service.get_user_tasks(user.email)
+                    if new_task_id not in user_tasks:
+                        continue
                     channel = f"task_progress:{new_task_id}"
                     if channel not in subscribed_channels:
                         await pubsub.subscribe(channel)