fix: address final-review hygiene items (pydantic config, cookie alias, argon2 exception scope)

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013PphXq1s43DNRj1uWKGXof

backend/app/routes/auth.py

@@ -65,7 +65,7 @@ async def login(payload: LoginRequest, response: Response, db: AsyncSession = De
 @router.post("/logout", status_code=status.HTTP_204_NO_CONTENT)
 async def logout(
     response: Response,
-    qm_session: str | None = Cookie(default=None),
+    qm_session: str | None = Cookie(default=None, alias=SESSION_COOKIE_NAME),
     db: AsyncSession = Depends(get_db),
 ):
     if qm_session is not None: