Remove .env from tracking, keep .env.example as template

- .env contains sensitive information and should not be tracked
- Users should copy .env.example to .env and configure their own values
- .env is already in .gitignore to prevent future tracking

.env

-# ========================================
-# 绩效计分系统 - 环境变量配置
-# ========================================
-
-# ==================== 前端配置 ====================
-# API 服务地址 - 前端访问后端API的地址
-VITE_API_BASE_URL=http://localhost:8000
-
-# ==================== 后端配置 ====================
-# 数据库配置
-DATABASE_URL=postgresql://performance_user:performance_pass@localhost:5431/performance_db
-
-# API 服务配置
-API_HOST=0.0.0.0
-API_PORT=8000
-
-# CORS 配置（允许的前端域名）
-CORS_ORIGINS=http://localhost:5173,http://localhost:8080,http://localhost:4001
-
-# 安全配置
-SECRET_KEY=your-secret-key-here-change-in-production
-
-# 日志配置
-LOG_LEVEL=INFO
-LOG_FILE=logs/api.log
-
-# 数据迁移配置
-MIGRATION_BATCH_SIZE=100