chore: unify data folder and update AI instructions (#58)

- Use single ./data folder for both dev and prod (removes ./backend/data)
- Update docker-compose.dev.yml to use ./data:/app/data
- Remove backend/data/ from .gitignore (only data/ needed)
- Add 'NEVER create PRs without permission' rule to copilot-instructions.md

.gitignore

@@ -34,7 +34,6 @@ coverage/
 *.db-journal
 *.db-wal
 *.db-shm
-backend/data/
 data/
 
 # ===================