feat: migrate configuration to .env environment variables

This commit implements a complete migration from JSON-based configuration
to .env environment variables, providing better security and flexibility.

Key Changes:
- Add python-dotenv dependency for environment variable support
- Create config/env_loader.py with type conversion utilities
- Add from_env() class methods to all config dataclasses
- Update ConfigLoader to prioritize environment variables
- Add save_to_env() method for .env file management
- Implement database connection factory pattern
- Add base DAO and connection classes for better abstraction
- Support both SQL Server and MySQL with unified interface
- Create migration script (scripts/migrate_to_env.py)
- Update GUI to read/write .env files
- Add comprehensive migration documentation

New Files:
- config/env_loader.py - Environment variable loader
- db/base_connection.py - Base database connection interface
- db/base_dao.py - Base DAO with common utilities
- db/connection_factory.py - Factory for creating connections
- db/mysql_connection.py - MySQL-specific connection
- db/sqlserver_connection.py - SQL Server-specific connection
- db/table_name_converter.py - SQL dialect converter
- scripts/migrate_to_env.py - Configuration migration tool
- docs/ENV_MIGRATION.md - Complete migration guide
- .env.example - Environment variable template

Testing:
- Verified MySQL connection (8.0.44)
- Tested all DAO operations
- Confirmed 150 tables accessible
- Validated configuration loading

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

db/mysql_connection.py

@@ -0,0 +1,142 @@
+"""
+MySQL 数据库连接组件
+
+提供 MySQL 数据库连接和查询接口
+"""
+
+import mysql.connector
+from mysql.connector import Error
+from typing import List, Dict, Any, Optional
+from db.base_connection import BaseDatabaseConnection
+
+
+class MySQLConnection(BaseDatabaseConnection):
+    """MySQL 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
+                - host: 服务器地址
+                - port: 端口号（默认 3306）
+                - database: 数据库名称
+                - username: 用户名
+                - password: 密码
+                - charset: 字符集（默认 utf8mb4）
+        """
+        super().__init__(config)
+
+    def connect(self):
+        """
+        建立数据库连接
+
+        Returns:
+            mysql.connector.connection.MySQLConnection: 数据库连接对象
+        """
+        if self.connection is not None:
+            return self.connection
+
+        try:
+            self.connection = mysql.connector.connect(
+                host=self.config.get('host', 'localhost'),
+                port=self.config.get('port', 3306),
+                database=self.config['database'],
+                user=self.config['username'],
+                password=self.config['password'],
+                charset=self.config.get('charset', 'utf8mb4'),
+                autocommit=False
+            )
+            print(
+                f"成功连接到 MySQL 数据库: {self.config.get('host', 'localhost')}"
+                f":{self.config.get('port', 3306)}/{self.config['database']}"
+            )
+            return self.connection
+        except Error as e:
+            print(f"MySQL 数据库连接失败: {e}")
+            raise
+
+    def disconnect(self):
+        """关闭数据库连接"""
+        if self.connection and self.connection.is_connected():
+            self.connection.close()
+            self.connection = None
+            print("MySQL 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 or not self.connection.is_connected():
+            self.connect()
+
+        cursor = None
+        try:
+            cursor = self.connection.cursor(dictionary=True)
+            if params:
+                cursor.execute(sql, params)
+            else:
+                cursor.execute(sql)
+
+            # 直接获取字典列表
+            results = cursor.fetchall()
+            return results
+
+        except Error as e:
+            print(f"查询执行失败: {e}")
+            raise
+        finally:
+            if cursor:
+                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 or not self.connection.is_connected():
+            self.connect()
+
+        cursor = None
+        try:
+            cursor = self.connection.cursor()
+            if params:
+                cursor.execute(sql, params)
+            else:
+                cursor.execute(sql)
+
+            self.connection.commit()
+            return cursor.rowcount
+
+        except Error as e:
+            self.connection.rollback()
+            print(f"执行失败，已回滚: {e}")
+            raise
+        finally:
+            if cursor:
+                cursor.close()
+
+    def get_placeholder(self) -> str:
+        """
+        获取参数占位符
+
+        Returns:
+            MySQL 使用 "%s" 作为参数占位符
+        """
+        return "%s"