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>

gui/config_manager.py

@@ -4,6 +4,7 @@
 配置管理器
 
 负责加载、保存和管理用户配置。
+支持从环境变量和 .env 文件加载配置。
 """
 import os
 from typing import TYPE_CHECKING
@@ -19,15 +20,17 @@ if TYPE_CHECKING:
 class ConfigManager:
     """配置管理器"""
 
-    def __init__(self, config_file: str = "config/user_settings.json"):
+    def __init__(self, config_file: str = "config/user_settings.json", use_env: bool = True):
         """
         初始化配置管理器
 
         Args:
-            config_file: 配置文件路径
+            config_file: 配置文件路径（向后兼容）
+            use_env: 是否使用环境变量，默认为 True
         """
         self.config_file = config_file
-        self.config: AppConfig = ConfigLoader.load(config_file)
+        self.use_env = use_env
+        self.config: AppConfig = ConfigLoader.load(config_file, use_env=use_env)
 
         # 验证配置
         errors = self.config.validate()
@@ -40,10 +43,16 @@ class ConfigManager:
         """
         保存配置到文件
 
+        如果使用环境变量，则保存到 .env 文件
+        否则保存到 JSON 文件（向后兼容）
+
         Returns:
             保存是否成功
         """
-        return ConfigLoader.save(self.config, self.config_file)
+        if self.use_env:
+            return ConfigLoader.save_to_env(self.config, ".env")
+        else:
+            return ConfigLoader.save(self.config, self.config_file)
 
     def get(self, key: str, default=None):
         """
@@ -90,7 +99,7 @@ class ConfigManager:
 
     def reset_to_defaults(self) -> None:
         """重置为默认配置"""
-        self.config = ConfigLoader.load("default")  # 重新加载默认配置
+        self.config = AppConfig.from_env()  # 重新从环境变量加载默认配置
         self.save()
 
     @property