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_dao.py

@@ -0,0 +1,100 @@
+"""
+DAO 基类
+
+提供数据访问对象的通用方法和辅助函数
+"""
+
+from typing import Optional
+from config.schema import DatabaseType
+from db.base_connection import BaseDatabaseConnection
+from db.connection import get_connection
+from db.table_name_converter import TableNameConverter
+
+
+class BaseDAO:
+    """数据访问对象基类"""
+
+    def __init__(self):
+        """初始化 DAO"""
+        self.db: Optional[BaseDatabaseConnection] = None
+        # 从配置文件加载数据库类型
+        from config.loader import ConfigLoader
+        app_config = ConfigLoader.load()
+        self._db_type = app_config.database.db_type
+
+    def __enter__(self):
+        """进入上下文管理器，建立数据库连接"""
+        self.db = get_connection()
+        self.db.connect()
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        """退出上下文管理器，关闭数据库连接"""
+        if self.db:
+            self.db.disconnect()
+
+    def close(self):
+        """关闭数据库连接"""
+        if self.db:
+            self.db.disconnect()
+
+    def _convert_sql(self, sql: str) -> str:
+        """
+        根据当前数据库类型转换 SQL 语句中的表名
+
+        Args:
+            sql: 原始 SQL 语句（SQL Server 格式）
+
+        Returns:
+            转换后的 SQL 语句
+        """
+        if self._db_type == DatabaseType.MYSQL:
+            # SQL Server → MySQL
+            return TableNameConverter.convert_sql(sql, 'mysql')
+        return sql
+
+    def _get_placeholder(self) -> str:
+        """
+        获取当前数据库类型的参数占位符
+
+        Returns:
+            SQL Server 返回 "?"，MySQL 返回 "%s"
+        """
+        if self._db_type == DatabaseType.MYSQL:
+            return "%s"
+        return "?"
+
+    def _build_placeholders(self, count: int) -> str:
+        """
+        构建参数占位符字符串
+
+        Args:
+            count: 占位符数量
+
+        Returns:
+            占位符字符串，如 "?, ?, ?" 或 "%s, %s, %s"
+        """
+        placeholder = self._get_placeholder()
+        return ", ".join([placeholder for _ in range(count)])
+
+    def _build_in_clause_placeholders(self, count: int) -> str:
+        """
+        构建 IN 子句的参数占位符字符串
+
+        Args:
+            count: 占位符数量
+
+        Returns:
+            IN 子句占位符字符串，如 "?, ?, ?" 或 "%s, %s, %s"
+        """
+        placeholder = self._get_placeholder()
+        return ", ".join([placeholder for _ in range(count)])
+
+    def _get_connection(self):
+        """
+        获取数据库连接
+
+        Returns:
+            数据库连接对象
+        """
+        return get_connection()