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/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