feat: add MySQL database support alongside SQL Server

This commit implements multi-database support, allowing the system to switch
between SQL Server and MySQL databases seamlessly.

## New Features
- Database type selection (SQL Server or MySQL) via configuration
- Automatic table name conversion between formats ([dbo].[table] → dbo_table)
- Automatic parameter placeholder handling (? for SQL Server, %s for MySQL)
- GUI settings tab now includes database type dropdown and MySQL configuration

## Database Abstraction Layer
- db/base_connection.py: Abstract base class for database connections
- db/sqlserver_connection.py: SQL Server implementation
- db/mysql_connection.py: MySQL implementation using mysql-connector-python
- db/connection_factory.py: Factory pattern for creating connections
- db/table_name_converter.py: Table name format conversion utility

## DAO Base Class
- db/base_dao.py: Base DAO with helper methods for SQL conversion and placeholders

## Updated Components
- config/schema.py: Extended with DatabaseType enum and MySQL/SQLServer config classes
- config/defaults.py: Added MySQL default configuration
- config/loader.py: Updated to handle new database structure
- db/connection.py: Refactored to use factory pattern and load user config
- All DAO files: Updated to inherit from BaseDAO with automatic conversion

## Dependencies
- Added mysql-connector-python>=8.0.0 to requirements.txt

## Configuration
To use MySQL, set db_type to "mysql" in config/user_settings.json:
{
  "database": {
    "db_type": "mysql",
    "mysql": {
      "host": "192.168.31.83",
      "port": 3306,
      "database": "BLD_DB",
      "username": "remote_user",
      "password": "3.1415926Beeke"
    }
  }
}

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

db/base_dao.py

@@ -0,0 +1,100 @@
+"""
+DAO 基类
+
+提供数据访问对象的通用方法和辅助函数
+"""
+
+from typing import Optional
+from config.schema import DatabaseType
+from db.base_connection import BaseDatabaseConnection
+from db.connection import get_connection
+from db.table_name_converter import TableNameConverter
+
+
+class BaseDAO:
+    """数据访问对象基类"""
+
+    def __init__(self):
+        """初始化 DAO"""
+        self.db: Optional[BaseDatabaseConnection] = None
+        # 从配置文件加载数据库类型
+        from config.loader import ConfigLoader
+        app_config = ConfigLoader.load()
+        self._db_type = app_config.database.db_type
+
+    def __enter__(self):
+        """进入上下文管理器，建立数据库连接"""
+        self.db = get_connection()
+        self.db.connect()
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        """退出上下文管理器，关闭数据库连接"""
+        if self.db:
+            self.db.disconnect()
+
+    def close(self):
+        """关闭数据库连接"""
+        if self.db:
+            self.db.disconnect()
+
+    def _convert_sql(self, sql: str) -> str:
+        """
+        根据当前数据库类型转换 SQL 语句中的表名
+
+        Args:
+            sql: 原始 SQL 语句（SQL Server 格式）
+
+        Returns:
+            转换后的 SQL 语句
+        """
+        if self._db_type == DatabaseType.MYSQL:
+            # SQL Server → MySQL
+            return TableNameConverter.convert_sql(sql, 'mysql')
+        return sql
+
+    def _get_placeholder(self) -> str:
+        """
+        获取当前数据库类型的参数占位符
+
+        Returns:
+            SQL Server 返回 "?"，MySQL 返回 "%s"
+        """
+        if self._db_type == DatabaseType.MYSQL:
+            return "%s"
+        return "?"
+
+    def _build_placeholders(self, count: int) -> str:
+        """
+        构建参数占位符字符串
+
+        Args:
+            count: 占位符数量
+
+        Returns:
+            占位符字符串，如 "?, ?, ?" 或 "%s, %s, %s"
+        """
+        placeholder = self._get_placeholder()
+        return ", ".join([placeholder for _ in range(count)])
+
+    def _build_in_clause_placeholders(self, count: int) -> str:
+        """
+        构建 IN 子句的参数占位符字符串
+
+        Args:
+            count: 占位符数量
+
+        Returns:
+            IN 子句占位符字符串，如 "?, ?, ?" 或 "%s, %s, %s"
+        """
+        placeholder = self._get_placeholder()
+        return ", ".join([placeholder for _ in range(count)])
+
+    def _get_connection(self):
+        """
+        获取数据库连接
+
+        Returns:
+            数据库连接对象
+        """
+        return get_connection()