refactor(database): 阶段一 - 创建新架构基础

- 创建分层目录结构 (core/api/optimization/config/utils)
- 实现核心层: engine.py, session.py
- 实现配置层: database_config.py
- 实现工具层: exceptions.py
- 迁移连接池管理器到优化层
- 添加详细的重构计划文档

src/common/database/core/engine.py

@@ -0,0 +1,141 @@
+"""数据库引擎管理
+
+单一职责：创建和管理SQLAlchemy异步引擎
+"""
+
+import asyncio
+from typing import Optional
+
+from sqlalchemy import text
+from sqlalchemy.ext.asyncio import AsyncEngine, create_async_engine
+
+from src.common.logger import get_logger
+
+from ..config.database_config import get_database_config
+from ..utils.exceptions import DatabaseInitializationError
+
+logger = get_logger("database.engine")
+
+# 全局引擎实例
+_engine: Optional[AsyncEngine] = None
+_engine_lock: Optional[asyncio.Lock] = None
+
+
+async def get_engine() -> AsyncEngine:
+    """获取全局数据库引擎（单例模式）
+    
+    Returns:
+        AsyncEngine: SQLAlchemy异步引擎
+        
+    Raises:
+        DatabaseInitializationError: 引擎初始化失败
+    """
+    global _engine, _engine_lock
+    
+    # 快速路径：引擎已初始化
+    if _engine is not None:
+        return _engine
+    
+    # 延迟创建锁（避免在导入时创建）
+    if _engine_lock is None:
+        _engine_lock = asyncio.Lock()
+    
+    # 使用锁保护初始化过程
+    async with _engine_lock:
+        # 双重检查锁定模式
+        if _engine is not None:
+            return _engine
+        
+        try:
+            config = get_database_config()
+            
+            logger.info(f"正在初始化 {config.db_type.upper()} 数据库引擎...")
+            
+            # 创建异步引擎
+            _engine = create_async_engine(
+                config.url,
+                **config.engine_kwargs
+            )
+            
+            # SQLite特定优化
+            if config.db_type == "sqlite":
+                await _enable_sqlite_optimizations(_engine)
+            
+            logger.info(f"✅ {config.db_type.upper()} 数据库引擎初始化成功")
+            return _engine
+        
+        except Exception as e:
+            logger.error(f"❌ 数据库引擎初始化失败: {e}", exc_info=True)
+            raise DatabaseInitializationError(f"引擎初始化失败: {e}") from e
+
+
+async def close_engine():
+    """关闭数据库引擎
+    
+    释放所有连接池资源
+    """
+    global _engine
+    
+    if _engine is not None:
+        logger.info("正在关闭数据库引擎...")
+        await _engine.dispose()
+        _engine = None
+        logger.info("✅ 数据库引擎已关闭")
+
+
+async def _enable_sqlite_optimizations(engine: AsyncEngine):
+    """启用SQLite性能优化
+    
+    优化项：
+    - WAL模式：提高并发性能
+    - NORMAL同步：平衡性能和安全性
+    - 启用外键约束
+    - 设置busy_timeout：避免锁定错误
+    
+    Args:
+        engine: SQLAlchemy异步引擎
+    """
+    try:
+        async with engine.begin() as conn:
+            # 启用WAL模式
+            await conn.execute(text("PRAGMA journal_mode = WAL"))
+            # 设置适中的同步级别
+            await conn.execute(text("PRAGMA synchronous = NORMAL"))
+            # 启用外键约束
+            await conn.execute(text("PRAGMA foreign_keys = ON"))
+            # 设置busy_timeout，避免锁定错误
+            await conn.execute(text("PRAGMA busy_timeout = 60000"))
+            # 设置缓存大小（10MB）
+            await conn.execute(text("PRAGMA cache_size = -10000"))
+            # 临时存储使用内存
+            await conn.execute(text("PRAGMA temp_store = MEMORY"))
+        
+        logger.info("✅ SQLite性能优化已启用 (WAL模式 + 并发优化)")
+    
+    except Exception as e:
+        logger.warning(f"⚠️ SQLite性能优化失败: {e}，将使用默认配置")
+
+
+async def get_engine_info() -> dict:
+    """获取引擎信息（用于监控和调试）
+    
+    Returns:
+        dict: 引擎信息字典
+    """
+    try:
+        engine = await get_engine()
+        
+        info = {
+            "name": engine.name,
+            "driver": engine.driver,
+            "url": str(engine.url).replace(str(engine.url.password or ""), "***"),
+            "pool_size": getattr(engine.pool, "size", lambda: None)(),
+            "pool_checked_out": getattr(engine.pool, "checked_out", lambda: 0)(),
+            "pool_overflow": getattr(engine.pool, "overflow", lambda: 0)(),
+        }
+        
+        return info
+    
+    except Exception as e:
+        logger.error(f"获取引擎信息失败: {e}")
+        return {}