fix: UploadFile ForwardRef + WebSocket 403 — root cause fixed

1. plugin_error_handler.py: Remove _UploadFile alias, import UploadFile directly
   so FastAPI can resolve ForwardRef('UploadFile') in the wrapper's namespace.
   Also import WebSocket for ForwardRef resolution.

2. main.py: Skip WebSocket routes in wrap_plugin_route — WebSocket endpoints
   must not be wrapped (different protocol, no JSONResponse on error)

app/core/plugin_error_handler.py

@@ -2,7 +2,8 @@
 import logging
 import functools
 import inspect
-from fastapi import UploadFile as _UploadFile  # noqa: F401 — needed for ForwardRef resolution
+from fastapi import UploadFile  # noqa: F401 — needed for ForwardRef resolution
+from fastapi import WebSocket  # noqa: F401 — needed for ForwardRef resolution of WebSocket params
 from fastapi.responses import JSONResponse
 
 logger = logging.getLogger(__name__)