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/sqlserver_connection.py

@@ -0,0 +1,147 @@
+"""
+SQL Server 数据库连接组件
+
+提供 SQL Server 数据库连接和查询接口
+"""
+
+import pyodbc
+from typing import List, Dict, Any, Optional
+from db.base_connection import BaseDatabaseConnection
+
+
+class SQLServerConnection(BaseDatabaseConnection):
+    """SQL Server 数据库连接类"""
+
+    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
+                - server: 服务器地址
+                - database: 数据库名称
+                - username: 用户名
+                - password: 密码
+                - driver: ODBC 驱动名称
+                - trust_server_certificate: 是否信任服务器证书
+        """
+        super().__init__(config)
+
+    def connect(self) -> pyodbc.Connection:
+        """
+        建立数据库连接
+
+        Returns:
+            pyodbc.Connection: 数据库连接对象
+        """
+        if self.connection is not None:
+            return self.connection
+
+        # 构建连接字符串
+        driver = self.config.get('driver', 'ODBC Driver 18 for SQL Server')
+        conn_str = (
+            f"DRIVER={{{driver}}};"
+            f"SERVER={self.config['server']};"
+            f"DATABASE={self.config['database']};"
+            f"UID={self.config['username']};"
+            f"PWD={self.config['password']};"
+            f"TrustServerCertificate={self.config.get('trust_server_certificate', 'yes')};"
+        )
+
+        try:
+            self.connection = pyodbc.connect(conn_str)
+            print(
+                f"成功连接到 SQL Server 数据库: {self.config['server']}/{self.config['database']}"
+            )
+            return self.connection
+        except pyodbc.Error as e:
+            print(f"SQL Server 数据库连接失败: {e}")
+            raise
+
+    def disconnect(self):
+        """关闭数据库连接"""
+        if self.connection:
+            self.connection.close()
+            self.connection = None
+            print("SQL Server 数据库连接已关闭")
+
+    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
+        """
+        if not self.connection:
+            self.connect()
+
+        cursor = self.connection.cursor()
+
+        try:
+            if params:
+                cursor.execute(sql, params)
+            else:
+                cursor.execute(sql)
+
+            # 获取列名
+            columns = [column[0] for column in cursor.description]
+
+            # 将结果转换为字典列表
+            results = []
+            for row in cursor.fetchall():
+                results.append(dict(zip(columns, row)))
+
+            return results
+
+        except pyodbc.Error as e:
+            print(f"查询执行失败: {e}")
+            raise
+        finally:
+            cursor.close()
+
+    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
+        """
+        if not self.connection:
+            self.connect()
+
+        cursor = self.connection.cursor()
+
+        try:
+            if params:
+                cursor.execute(sql, params)
+            else:
+                cursor.execute(sql)
+
+            self.connection.commit()
+            return cursor.rowcount
+
+        except pyodbc.Error as e:
+            self.connection.rollback()
+            print(f"执行失败，已回滚: {e}")
+            raise
+        finally:
+            cursor.close()
+
+    def get_placeholder(self) -> str:
+        """
+        获取参数占位符
+
+        Returns:
+            SQL Server 使用 "?" 作为参数占位符
+        """
+        return "?"