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

docker-compose.dev.yml

@@ -6,7 +6,7 @@ services:
     volumes:
       - ./backend:/app
       - backend_node_modules:/app/node_modules
-      - ./backend/data:/app/data
+      - ./data:/app/data
     env_file:
       - .env
     ports: