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/share_link.py

@@ -1,6 +1,8 @@
 """ShareLink model for board sharing."""
+
 import uuid
 from datetime import datetime
+
 from sqlalchemy import Boolean, Column, DateTime, ForeignKey, Integer, String
 from sqlalchemy.dialects.postgresql import UUID
 from sqlalchemy.orm import relationship
@@ -29,4 +31,3 @@ class ShareLink(Base):
 
     def __repr__(self) -> str:
         return f"<ShareLink(id={self.id}, token={self.token[:8]}...)>"
-