feat(settings): 公司信息配置与 logo 管理（public 端点供登录页）

- system_settings 单行 CRUD：GET/PUT（AdminUser）+ GET /public（无鉴权，
  SettingPublicOut 白名单仅 company_name/logo_url 两字段，短缓存 300s）
- logo 上传：后缀+magic bytes+2MB 三重校验，固定名覆盖落盘 uploads/logo/
  （不使用上传方文件名，杜绝路径拼接）；旧扩展名文件清理；文件流端点
  no-cache，URL 带 update_time 版本参数破缓存
- logo 存目录不存库：图片入 DB 让每次读取与 mysqldump 放大传输且无法
  独立替换；DB 只持一行相对路径
- 新增依赖 python-multipart（FastAPI UploadFile 必备传输层）；.gitignore
  增 backend/uploads/ 与 db/backups/
- 集成 11 例：单行语义/白名单字段/三重校验/越权 403/匿名 401/审计留痕

Co-Authored-By: Claude Code <noreply@anthropic.com>

backend/app/api/router.py

@@ -19,6 +19,7 @@ from app.api.v1.system import business_roles as system_business_roles
 from app.api.v1.system import logs as system_logs
 from app.api.v1.system import params as system_params
 from app.api.v1.system import roles as system_roles
+from app.api.v1.system import settings as system_settings
 from app.api.v1.system import users as system_users
 
 api_router = APIRouter()
@@ -36,6 +37,7 @@ system_router.include_router(system_users.router)
 system_router.include_router(system_roles.router)
 system_router.include_router(system_business_roles.router)
 system_router.include_router(system_params.router)
+system_router.include_router(system_settings.router)
 system_router.include_router(system_logs.operation_router)
 system_router.include_router(system_logs.login_router)
 v1_router.include_router(system_router)