feat: add MySQL database support with SQL translation

- Add MySQLConnection class with automatic SQL Server to MySQL translation
- Add connection factory to support both SQL Server and MySQL
- Update config schema to support MySQL configuration (host, port, db_type)
- Update default config to use MySQL (localhost:3306)
- Translate table names: [schema].[table] -> schema_table
- Translate placeholders: ? -> %s
- Translate MERGE statements to INSERT ... ON DUPLICATE KEY UPDATE

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

db/connection_factory.py

@@ -0,0 +1,40 @@
+"""
+数据库连接工厂
+
+根据配置返回 MySQL 或 SQL Server 连接实例。
+"""
+
+from typing import Optional
+from db.connection import DatabaseConnection
+from db.mysql_connection import MySQLConnection
+from config.defaults import DEFAULT_APP_CONFIG
+
+
+def get_connection(db_type: Optional[str] = None):
+    """
+    根据配置返回 MySQL 或 SQL Server 连接
+
+    Args:
+        db_type: 数据库类型，"mysql" 或 "sqlserver"。
+                 如果为 None，则从 DEFAULT_APP_CONFIG 读取配置
+
+    Returns:
+        DatabaseConnection 或 MySQLConnection 实例
+
+    Example:
+        >>> # 使用默认配置
+        >>> conn = get_connection()
+
+        >>> # 强制使用 MySQL
+        >>> conn = get_connection("mysql")
+
+        >>> # 强制使用 SQL Server
+        >>> conn = get_connection("sqlserver")
+    """
+    if db_type is None:
+        db_type = DEFAULT_APP_CONFIG.database.db_type
+
+    if db_type == "mysql":
+        return MySQLConnection()
+
+    return DatabaseConnection()