refactor: configure pre-commit and CI/CD pipeline

- Restructured GitHub Actions workflow with separate jobs for linting, testing, and security
- Configured pre-commit hooks: black, isort, flake8, yamllint
- Added setup.cfg for centralized configuration
- Relaxed flake8 rules (B008, D* docstrings) for FastAPI compatibility
- Removed bandit (pbr dependency issue) - can be added later
- All pre-commit checks now passing

backend/app/db.py

@@ -1,5 +1,5 @@
 from sqlalchemy import create_engine
-from sqlalchemy.orm import sessionmaker, DeclarativeBase
+from sqlalchemy.orm import DeclarativeBase, sessionmaker
 
 # SQLite local file. In CI, you can swap to Postgres later.
 DATABASE_URL = "sqlite:///./taskmanager.db"