✨(settings) disable file upload by default & max count

Add FILE_UPLOAD_ENABLED setting (default to False, to avoid
a breaking change). Also adds a max_count_by_user sub setting
to restrict the number of uploaded files per user.

src/backend/core/api/feature_flag.py

@@ -13,6 +13,7 @@ class FeatureFlag:
         "recording": "RECORDING_ENABLE",
         "storage_event": "RECORDING_STORAGE_EVENT_ENABLE",
         "subtitle": "ROOM_SUBTITLE_ENABLED",
+        "file_upload": "FILE_UPLOAD_ENABLED",
     }
 
     @classmethod