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/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 "?"