Improve file listing and fix notification metadata field

Backend:
- Optimize file listing for non-superusers with dedicated CRUD methods
- Add get_visible_for_user and count_visible_for_user for efficient queries
- Move /allowed-types/ and /max-size/ routes before /{file_id} for proper matching
- Rename notification 'metadata' field to 'extra_data' for clarity
- Fix settings export to use get_value() method

Frontend:
- Update NotificationItem interface to use extra_data field

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/api/v1/notifications.py

@@ -23,12 +23,12 @@ router = APIRouter()
 
 def serialize_notification(db_obj) -> dict:
     """Serialize notification for response."""
-    metadata = None
-    if db_obj.metadata:
+    extra_data = None
+    if db_obj.extra_data:
         try:
-            metadata = json.loads(db_obj.metadata)
+            extra_data = json.loads(db_obj.extra_data)
         except json.JSONDecodeError:
-            metadata = None
+            extra_data = None
 
     return {
         "id": db_obj.id,
@@ -37,7 +37,7 @@ def serialize_notification(db_obj) -> dict:
         "message": db_obj.message,
         "type": db_obj.type,
         "link": db_obj.link,
-        "metadata": metadata,
+        "extra_data": extra_data,
         "is_read": db_obj.is_read,
         "created_at": db_obj.created_at,
         "read_at": db_obj.read_at
@@ -239,6 +239,6 @@ def broadcast_notification(
         message=notification_in.message,
         type=notification_in.type,
         link=notification_in.link,
-        metadata=notification_in.metadata
+        extra_data=notification_in.extra_data
     )
     return {"sent_to": count}