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/tests/test_database.py

@@ -0,0 +1,26 @@
+from pathlib import Path
+
+def test_database_initialization(db_path):
+    """测试数据库初始化."""
+    from app.database import init_db, get_engine, Base
+
+    init_db(db_path)
+
+    assert db_path.exists()
+
+    engine = get_engine()
+    assert engine is not None
+
+    # 创建所有表
+    Base.metadata.create_all(engine)
+    assert True  # 如果没有异常则成功
+
+
+def test_get_session(db_path):
+    """测试获取数据库会话."""
+    from app.database import init_db, get_db
+
+    init_db(db_path)
+
+    with get_db(db_path) as session:
+        assert session is not None