feat: add unified linting scripts and git hooks for code quality enforcement

- Introduced `lint` and `lint-fix` applications in `flake.nix` for unified linting of backend (Python) and frontend (TypeScript/Svelte) code.
- Added `scripts/lint.sh` for manual linting execution.
- Created `scripts/install-hooks.sh` to set up git hooks for automatic linting before commits and optional tests before pushes.
- Updated `README.md` with instructions for using the new linting features and git hooks.

backend/app/database/models/__init__.py

@@ -1,11 +1,12 @@
 """Database models."""
-from app.database.models.user import User
+
 from app.database.models.board import Board
-from app.database.models.image import Image
 from app.database.models.board_image import BoardImage
-from app.database.models.group import Group
-from app.database.models.share_link import ShareLink
 from app.database.models.comment import Comment
+from app.database.models.group import Group
+from app.database.models.image import Image
+from app.database.models.share_link import ShareLink
+from app.database.models.user import User
 
 __all__ = [
     "User",