chore: CI/CD + 前端测试 + 安全加固 + 限流完善

- 新增 .gitlab-ci.yml (lint/test/build 三阶段)
- 新增前端测试: taskStageMapper (109), api.ts (36), AuthContext (16)
- 修复旧测试: Sidebar 导航文案、MobileLayout padding 值
- python-jose → PyJWT 消除 ecdsa CVE 漏洞
- 限流中间件增加 5 个敏感端点精细限流 + 标准限流头

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/services/auth.py

@@ -4,7 +4,8 @@
 from datetime import datetime, timedelta
 from typing import Optional
 import secrets
-from jose import jwt, JWTError
+import jwt
+from jwt.exceptions import PyJWTError as JWTError
 from passlib.context import CryptContext
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy import select