backend v2.1: 公告管理功能 + 系统重构

- 新增 Announcement 数据模型，支持公告的增删改查
- 后台管理新增"公告管理"Tab（创建/编辑/删除/启用禁用）
- 客户端 /api/announcement 改为从数据库读取
- 账号服务重构，新增无感换号、自动分析等功能
- 新增后台任务调度器、数据库迁移脚本
- Schema/Service/Config 全面升级至 v2.1

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

backend/app/services/auth_service.py

@@ -1,4 +1,4 @@
-from datetime import datetime, timedelta
+from datetime import datetime, timedelta, timezone
 from typing import Optional
 from jose import JWTError, jwt
 from passlib.context import CryptContext
@@ -21,9 +21,9 @@ def get_password_hash(password: str) -> str:
 def create_access_token(data: dict, expires_delta: Optional[timedelta] = None) -> str:
     to_encode = data.copy()
     if expires_delta:
-        expire = datetime.utcnow() + expires_delta
+        expire = datetime.now(timezone.utc) + expires_delta
     else:
-        expire = datetime.utcnow() + timedelta(minutes=settings.ACCESS_TOKEN_EXPIRE_MINUTES)
+        expire = datetime.now(timezone.utc) + timedelta(minutes=settings.ACCESS_TOKEN_EXPIRE_MINUTES)
     to_encode.update({"exp": expire})
     encoded_jwt = jwt.encode(to_encode, settings.SECRET_KEY, algorithm=settings.ALGORITHM)
     return encoded_jwt