feat: 完成管理员密码登录修复和项目清理

- 修复BCryptPasswordEncoder密码验证问题
- 实现密码设置提示弹窗功能（仅对无密码用户显示一次）
- 优化修改密码逻辑和验证流程
- 更新Welcome页面背景样式
- 清理临时SQL文件和测试代码
- 移动数据库备份文件到database/backups目录
- 删除不必要的MD文档和临时文件

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