refactor(auth): remove legacy RESTful user routes and migrate authentication to GraphQL context

- Rename get_current_user to get_current_user_id in auth/deps.py to clarify intent
- Remove legacy /me, /logout routes, and placeholder function from auth/__init__.py
- Remove dependency_overrides from app/main.py
- Update CustomContext to resolve and cache full User objects via db query

auth/deps.py

@@ -9,7 +9,8 @@ load_dotenv()
 JWT_SECRET = os.getenv("JWT_SECRET")
 JWT_ALGORITHM = os.getenv("JWT_ALGORITHM", "HS256")
 
-def get_current_user(request: Request):
+def get_current_user_id(request: Request):
+    """仅从 Cookie 中提取并解密出用户的 user_id (sub)"""
     token = request.cookies.get("access_token")
 
     if not token: