feat: add database module

- Add SQLAlchemy database module with DeclarativeBase
- Implement engine and session factory management
- Add context manager for database sessions
- Add database initialization script
- Update models/__init__.py to import Base from database
- Fix Python 3.8 compatibility issues (use Optional instead of |)
- Ensure SQLite database file is created on init_db

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -1,11 +1,3 @@
-"""ORM Models.
+from app.database import Base
 
-NOTE: This module is a placeholder until Task 2.1.
-The Base class is needed by conftest.py for database fixtures.
-"""
-
-from sqlalchemy.orm import DeclarativeBase
-
-class Base(DeclarativeBase):
-    """Base class for all ORM models."""
-    pass
+__all__ = ['Base']