fix: UploadFile ForwardRef error + WebSocket 403 CSRF block

1. plugin_error_handler.py: Remove return_annotation from copied signature
   to prevent FastAPI ForwardRef('UploadFile') resolution failure on routes
   with file upload endpoints (dms, calendar, mail, kommunikation, ai_assistant)

2. middleware.py: Skip CSRF check for WebSocket upgrade requests
   WebSocket connections use GET with upgrade header — should not be
   blocked by CSRF middleware

app/core/middleware.py

@@ -69,6 +69,10 @@ class CSRFMiddleware(BaseHTTPMiddleware):
     UNSAFE_METHODS = {"POST", "PATCH", "PUT", "DELETE"}
 
     async def dispatch(self, request: Request, call_next):
+        # Skip WebSocket upgrade requests — they use GET and are handled separately
+        if request.headers.get("upgrade", "").lower() == "websocket":
+            return await call_next(request)
+
         if request.method in self.UNSAFE_METHODS:
             # 1. Origin header check
             origin = request.headers.get("origin")