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/app.py

@@ -415,7 +415,7 @@ async def cancel_task(
     # Verify user owns this task
     user_tasks = task_service.get_user_tasks(user.email)
     if task_id not in user_tasks:
-        return {"success": False, "message": "Task not found or not owned by user"}
+        raise HTTPException(status_code=404, detail="Task not found or not owned by user")
 
     try:
         task_service.cancel_task(task_id, user_email=user.email)