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_connection.py

@@ -0,0 +1,84 @@
+"""
+数据库连接抽象基类
+
+定义数据库连接的通用接口
+"""
+
+from abc import ABC, abstractmethod
+from typing import List, Dict, Any, Optional
+
+
+class BaseDatabaseConnection(ABC):
+    """数据库连接抽象基类"""
+
+    def __init__(self, config: Optional[Dict[str, Any]] = None):
+        """
+        初始化数据库连接
+
+        Args:
+            config: 数据库配置字典
+        """
+        self.config = config or {}
+        self.connection = None
+
+    @abstractmethod
+    def connect(self):
+        """
+        建立数据库连接
+
+        Returns:
+            数据库连接对象
+        """
+        pass
+
+    @abstractmethod
+    def disconnect(self):
+        """关闭数据库连接"""
+        pass
+
+    @abstractmethod
+    def execute_query(self, sql: str, params: Optional[tuple] = None) -> List[Dict[str, Any]]:
+        """
+        执行查询语句并返回结果
+
+        Args:
+            sql: SQL 查询语句
+            params: 查询参数（可选）
+
+        Returns:
+            List[Dict[str, Any]]: 查询结果列表，每个元素为一行数据的字典
+        """
+        pass
+
+    @abstractmethod
+    def execute_update(self, sql: str, params: Optional[tuple] = None) -> int:
+        """
+        执行更新/插入/删除语句
+
+        Args:
+            sql: SQL 语句
+            params: 参数（可选）
+
+        Returns:
+            int: 受影响的行数
+        """
+        pass
+
+    def __enter__(self):
+        """支持 with 语句的上下文管理器入口"""
+        self.connect()
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        """支持 with 语句的上下文管理器出口"""
+        self.disconnect()
+
+    @abstractmethod
+    def get_placeholder(self) -> str:
+        """
+        获取参数占位符
+
+        Returns:
+            参数占位符字符串（SQL Server: "?" 或 MySQL: "%s"）
+        """
+        pass