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/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()