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/database.py

@@ -0,0 +1,73 @@
+import sqlalchemy
+from sqlalchemy import create_engine, text
+from sqlalchemy.orm import DeclarativeBase, sessionmaker, Session
+from pathlib import Path
+from contextlib import contextmanager
+from typing import Generator, Optional
+
+
+class Base(DeclarativeBase):
+    """SQLAlchemy 声明基类."""
+    pass
+
+
+_engine = None
+_session_factory = None
+
+
+def init_db(db_path: Path) -> None:
+    """
+    初始化数据库引擎和会话工厂.
+
+    Args:
+        db_path: SQLite 数据库文件路径
+    """
+    global _engine, _session_factory
+
+    # 确保父目录存在
+    db_path.parent.mkdir(parents=True, exist_ok=True)
+
+    # 创建引擎
+    _engine = create_engine(
+        f'sqlite:///{db_path}',
+        connect_args={'check_same_thread': False}
+    )
+    _session_factory = sessionmaker(bind=_engine, autocommit=False, autoflush=False)
+
+    # 确保 SQLite 数据库文件被创建（SQLite 是惰性创建的）
+    with _engine.connect() as conn:
+        conn.execute(text("SELECT 1"))
+
+
+def get_engine():
+    """获取数据库引擎."""
+    return _engine
+
+
+def get_session_factory():
+    """获取会话工厂."""
+    return _session_factory
+
+
+@contextmanager
+def get_db(db_path: Optional[Path] = None) -> Generator[Session, None, None]:
+    """
+    获取数据库会话.
+
+    Args:
+        db_path: 可选，用于初始化数据库
+
+    Yields:
+        SQLAlchemy 会话
+    """
+    if db_path and _engine is None:
+        init_db(db_path)
+
+    if _session_factory is None:
+        raise RuntimeError("Database not initialized. Call init_db() first.")
+
+    session = _session_factory()
+    try:
+        yield session
+    finally:
+        session.close()