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/table_name_converter.py

@@ -0,0 +1,140 @@
+"""
+表名转换工具
+
+处理 SQL Server 和 MySQL 之间的表名格式转换
+"""
+
+import re
+from typing import List
+
+
+class TableNameConverter:
+    """表名转换工具类"""
+
+    # 匹配 SQL Server 表名格式：[schema].[tablename] 或 [schema].[table name]
+    SQLSERVER_PATTERN = re.compile(r'\[([^\]]+)\]\.\[([^\]]+)\]')
+
+    @staticmethod
+    def to_mysql(table_name: str) -> str:
+        """
+        将 SQL Server 表名格式转换为 MySQL 格式
+
+        SQL Server: [schema].[tablename] → MySQL: schema_tablename
+        SQL Server: tablename → MySQL: dbo_tablename (默认 dbo)
+
+        Args:
+            table_name: SQL Server 格式的表名
+
+        Returns:
+            MySQL 格式的表名
+
+        Examples:
+            >>> TableNameConverter.to_mysql('[dbo].[BIPUsers]')
+            'dbo_BIPUsers'
+            >>> TableNameConverter.to_mysql('DiscreteMaterialPlanData')
+            'dbo_DiscreteMaterialPlanData'
+            >>> TableNameConverter.to_mysql('[productionContractData].[26年压力表合同数据]')
+            'productionContractData_26年压力表合同数据'
+        """
+        # 尝试匹配 [schema].[tablename] 格式
+        match = TableNameConverter.SQLSERVER_PATTERN.match(table_name.strip())
+        if match:
+            schema = match.group(1)
+            table = match.group(2)
+            return f"{schema}_{table}"
+
+        # 如果没有匹配到，使用默认 schema dbo
+        return f"dbo_{table_name}"
+
+    @staticmethod
+    def to_sqlserver(table_name: str) -> str:
+        """
+        将 MySQL 表名格式转换为 SQL Server 格式
+
+        MySQL: schema_tablename → SQL Server: [schema].[tablename]
+
+        Args:
+            table_name: MySQL 格式的表名
+
+        Returns:
+            SQL Server 格式的表名
+
+        Examples:
+            >>> TableNameConverter.to_sqlserver('dbo_BIPUsers')
+            '[dbo].[BIPUsers]'
+            >>> TableNameConverter.to_sqlserver('productionContractData_26年压力表合同数据')
+            '[productionContractData].[26年压力表合同数据]'
+        """
+        # 分割第一个下划线
+        parts = table_name.split('_', 1)
+        if len(parts) == 2:
+            schema = parts[0]
+            table = parts[1]
+            return f"[{schema}].[{table}]"
+
+        # 如果没有下划线，使用默认 schema dbo
+        return f"[dbo].[{table_name}]"
+
+    @staticmethod
+    def convert_sql(sql: str, db_type: str) -> str:
+        """
+        批量转换 SQL 语句中的表名
+
+        Args:
+            sql: SQL 语句
+            db_type: 目标数据库类型 ('sqlserver' 或 'mysql')
+
+        Returns:
+            转换后的 SQL 语句
+
+        Examples:
+            >>> sql = "SELECT * FROM [dbo].[BIPUsers] WHERE ID = ?"
+            >>> TableNameConverter.convert_sql(sql, 'mysql')
+            'SELECT * FROM dbo_BIPUsers WHERE ID = ?'
+        """
+        if db_type == 'mysql':
+            # SQL Server → MySQL
+            def replace_to_mysql(match):
+                schema = match.group(1)
+                table = match.group(2)
+                return f"{schema}_{table}"
+            result = TableNameConverter.SQLSERVER_PATTERN.sub(replace_to_mysql, sql)
+            return result
+        elif db_type == 'sqlserver':
+            # MySQL → SQL Server
+            # 首先查找可能的 MySQL 格式表名（schema_table 格式）
+            # 这是一个简化版本，可能无法处理所有边缘情况
+            result = sql
+            # 查找单词字符_单词字符 的模式（可能是表名）
+            mysql_pattern = re.compile(r'\b([a-zA-Z_][a-zA-Z0-9_]*)_([a-zA-Z0-9_\u4e00-\u9fff]+)\b')
+            matches = mysql_pattern.findall(result)
+            for schema, table in set(matches):
+                mysql_name = f"{schema}_{table}"
+                sqlserver_name = f"[{schema}].[{table}]"
+                result = result.replace(mysql_name, sqlserver_name)
+            return result
+        return sql
+
+    @staticmethod
+    def extract_table_names(sql: str) -> List[str]:
+        """
+        从 SQL 语句中提取所有表名
+
+        Args:
+            sql: SQL 语句
+
+        Returns:
+            表名列表
+        """
+        tables = []
+        # 查找 SQL Server 格式
+        sqlserver_matches = TableNameConverter.SQLSERVER_PATTERN.findall(sql)
+        for schema, table in sqlserver_matches:
+            tables.append(f"{schema}_{table}")
+
+        # 查找可能的 MySQL 格式
+        mysql_pattern = re.compile(r'\b[a-zA-Z_][a-zA-Z0-9_]*_[a-zA-Z0-9_]+\b')
+        mysql_matches = mysql_pattern.findall(sql)
+        tables.extend(mysql_matches)
+
+        return list(set(tables))