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()

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']

backend/init_db.py

@@ -0,0 +1,40 @@
+#!/usr/bin/env python3
+"""
+数据库初始化脚本.
+创建所有表和必要的目录.
+"""
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).parent))
+
+from app.config import get_settings
+from app.database import init_db
+from app.models import Base
+
+
+def main():
+    """初始化数据库."""
+    settings = get_settings()
+    print(f"初始化数据库: {settings.db_path}")
+
+    # 创建目录
+    settings.data_dir.mkdir(parents=True, exist_ok=True)
+    settings.ssh_keys_dir.mkdir(parents=True, exist_ok=True)
+    settings.repos_dir.mkdir(parents=True, exist_ok=True)
+
+    # 初始化数据库
+    init_db(settings.db_path)
+
+    # 创建所有表
+    from app.database import get_engine
+    Base.metadata.create_all(get_engine())
+
+    print("数据库初始化成功!")
+    print(f"  - 数据库: {settings.db_path}")
+    print(f"  - SSH 密钥: {settings.ssh_keys_dir}")
+    print(f"  - 仓库: {settings.repos_dir}")
+
+
+if __name__ == '__main__':
+    main()

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