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>

frontend/src/api/client.ts

@@ -244,7 +244,7 @@ export interface NotificationItem {
   message: string | null;
   type: 'info' | 'success' | 'warning' | 'error' | 'system';
   link: string | null;
-  metadata: Record<string, unknown> | null;
+  extra_data: Record<string, unknown> | null;
   is_read: boolean;
   created_at: string;
   read_at: string | null;