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>

config/env_loader.py

@@ -0,0 +1,201 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+"""
+环境变量加载器
+
+使用 python-dotenv 加载 .env 文件，并提供类型转换功能。
+"""
+import os
+from pathlib import Path
+from typing import Any, Optional, Type, TypeVar
+from dotenv import load_dotenv
+
+# 项目根目录
+PROJECT_ROOT = Path(__file__).parent.parent
+
+
+def load_env_file(env_file: Optional[str] = None) -> None:
+    """
+    加载 .env 文件
+
+    Args:
+        env_file: .env 文件路径，默认为项目根目录下的 .env
+    """
+    if env_file is None:
+        env_file = PROJECT_ROOT / ".env"
+    else:
+        env_file = Path(env_file)
+
+    load_dotenv(env_file)
+
+
+def get_env(key: str, default: Any = None) -> str:
+    """
+    获取环境变量
+
+    Args:
+        key: 环境变量名
+        default: 默认值
+
+    Returns:
+        环境变量值
+    """
+    return os.getenv(key, default)
+
+
+def get_env_bool(key: str, default: bool = False) -> bool:
+    """
+    获取布尔类型环境变量
+
+    Args:
+        key: 环境变量名
+        default: 默认值
+
+    Returns:
+        布尔值
+    """
+    value = os.getenv(key, "")
+    if not value:
+        return default
+    return value.lower() in ("true", "1", "yes", "on")
+
+
+def get_env_int(key: str, default: int = 0) -> int:
+    """
+    获取整数类型环境变量
+
+    Args:
+        key: 环境变量名
+        default: 默认值
+
+    Returns:
+        整数值
+    """
+    value = os.getenv(key, "")
+    if not value:
+        return default
+    try:
+        return int(value)
+    except ValueError:
+        return default
+
+
+def get_env_float(key: str, default: float = 0.0) -> float:
+    """
+    获取浮点数类型环境变量
+
+    Args:
+        key: 环境变量名
+        default: 默认值
+
+    Returns:
+        浮点数值
+    """
+    value = os.getenv(key, "")
+    if not value:
+        return default
+    try:
+        return float(value)
+    except ValueError:
+        return default
+
+
+def set_env(key: str, value: Any) -> None:
+    """
+    设置环境变量（仅在当前进程中有效）
+
+    Args:
+        key: 环境变量名
+        value: 环境变量值
+    """
+    os.environ[key] = str(value)
+
+
+def save_env_file(env_file: Optional[str] = None, env_dict: Optional[dict] = None) -> bool:
+    """
+    保存环境变量到 .env 文件
+
+    Args:
+        env_file: .env 文件路径，默认为项目根目录下的 .env
+        env_dict: 要保存的环境变量字典，如果为 None 则保存当前所有环境变量
+
+    Returns:
+        保存是否成功
+    """
+    if env_file is None:
+        env_file = PROJECT_ROOT / ".env"
+    else:
+        env_file = Path(env_file)
+
+    try:
+        # 确保目录存在
+        env_file.parent.mkdir(parents=True, exist_ok=True)
+
+        # 读取现有的 .env 文件以保留注释
+        existing_lines = []
+        if env_file.exists():
+            with open(env_file, "r", encoding="utf-8") as f:
+                existing_lines = f.readlines()
+
+        # 如果提供了 env_dict，则保存指定的环境变量
+        if env_dict is not None:
+            # 构建新的文件内容
+            new_content = []
+            processed_keys = set()
+
+            for line in existing_lines:
+                stripped = line.strip()
+                # 保留注释和空行
+                if not stripped or stripped.startswith("#"):
+                    new_content.append(line)
+                # 更新已存在的键值对
+                elif "=" in stripped and not stripped.startswith("#"):
+                    key = stripped.split("=")[0].strip()
+                    if key in env_dict:
+                        value = env_dict[key]
+                        # 处理布尔值的格式
+                        if isinstance(value, bool):
+                            value = "true" if value else "false"
+                        new_content.append(f"{key}={value}\n")
+                        processed_keys.add(key)
+                    else:
+                        new_content.append(line)
+
+            # 添加新的键值对
+            for key, value in env_dict.items():
+                if key not in processed_keys:
+                    # 处理布尔值的格式
+                    if isinstance(value, bool):
+                        value = "true" if value else "false"
+                    new_content.append(f"{key}={value}\n")
+
+            # 写入文件
+            with open(env_file, "w", encoding="utf-8") as f:
+                f.writelines(new_content)
+        else:
+            # 如果没有提供 env_dict，则不执行任何操作
+            # 因为保存所有环境变量可能会包含系统变量
+            return False
+
+        return True
+    except IOError as e:
+        print(f"保存 .env 文件失败: {e}")
+        return False
+
+
+def update_env_file(env_file: Optional[str] = None, **kwargs) -> bool:
+    """
+    更新 .env 文件中的特定环境变量
+
+    Args:
+        env_file: .env 文件路径
+        **kwargs: 要更新的环境变量键值对
+
+    Returns:
+        更新是否成功
+    """
+    return save_env_file(env_file, kwargs)
+
+
+# 自动加载 .env 文件
+load_env_file()