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/connection_factory.py

@@ -0,0 +1,90 @@
+"""
+数据库连接工厂
+
+根据配置创建对应数据库类型的连接实例
+"""
+
+from typing import Dict, Any, Optional
+from config.schema import DatabaseType
+from db.base_connection import BaseDatabaseConnection
+from db.sqlserver_connection import SQLServerConnection
+from db.mysql_connection import MySQLConnection
+
+
+class ConnectionFactory:
+    """数据库连接工厂类"""
+
+    @staticmethod
+    def create_connection(
+        db_type: DatabaseType,
+        config: Optional[Dict[str, Any]] = None
+    ) -> BaseDatabaseConnection:
+        """
+        根据数据库类型创建对应的连接实例
+
+        Args:
+            db_type: 数据库类型（SQLSERVER 或 MYSQL）
+            config: 数据库配置字典
+
+        Returns:
+            对应数据库的连接实例
+
+        Raises:
+            ValueError: 不支持的数据库类型
+        """
+        if db_type == DatabaseType.SQLSERVER:
+            return SQLServerConnection(config)
+        elif db_type == DatabaseType.MYSQL:
+            return MySQLConnection(config)
+        else:
+            raise ValueError(f"不支持的数据库类型: {db_type}")
+
+    @staticmethod
+    def create_from_config(database_config) -> BaseDatabaseConnection:
+        """
+        从 DatabaseConfig 配置对象创建连接
+
+        Args:
+            database_config: DatabaseConfig 配置对象
+
+        Returns:
+            对应数据库的连接实例
+
+        Raises:
+            ValueError: 不支持的数据库类型
+        """
+        db_type = database_config.db_type
+
+        if db_type == DatabaseType.SQLSERVER:
+            # 构建 SQL Server 配置字典
+            config = {
+                'server': database_config.server,
+                'database': database_config.database,
+                'username': database_config.username,
+                'password': database_config.password,
+            }
+            if database_config.sqlserver:
+                config['driver'] = database_config.sqlserver.driver
+                config['trust_server_certificate'] = (
+                    database_config.sqlserver.trust_server_certificate
+                )
+            return SQLServerConnection(config)
+
+        elif db_type == DatabaseType.MYSQL:
+            # 构建 MySQL 配置字典
+            config = {
+                'database': database_config.database,
+                'username': database_config.username,
+                'password': database_config.password,
+            }
+            if database_config.mysql:
+                config['host'] = database_config.mysql.host
+                config['port'] = database_config.mysql.port
+                config['charset'] = database_config.mysql.charset
+            else:
+                # 回退到 server 字段（兼容旧配置）
+                config['host'] = database_config.server
+            return MySQLConnection(config)
+
+        else:
+            raise ValueError(f"不支持的数据库类型: {db_type}")