Move auth session signing into auth_service.login

backend/app/routers/auth.py

@@ -20,7 +20,6 @@ from app.dependencies import (
     SettingsDep,
 )
 from app.models.auth import LoginRequest, LoginResponse, LogoutResponse
-from app.services.auth_service import sign_session_token
 from app.utils.constants import SESSION_COOKIE_NAME
 
 log: structlog.stdlib.BoundLogger = structlog.get_logger()
@@ -59,10 +58,11 @@ async def login(
         HTTPException: 401 if the password is incorrect.
     """
     try:
-        session = await auth_service.login(
+        signed_token, expires_at = await auth_service.login(
             db,
             password=body.password,
             session_duration_minutes=settings.session_duration_minutes,
+            session_secret=settings.session_secret,
             session_repo=session_repo,
         )
     except ValueError as exc:
@@ -71,10 +71,6 @@ async def login(
             detail=str(exc),
         ) from exc
 
-    signed_token = sign_session_token(
-        session.token,
-        settings.session_secret,
-    )
     response.set_cookie(
         key=SESSION_COOKIE_NAME,
         value=signed_token,
@@ -83,7 +79,7 @@ async def login(
         secure=settings.session_cookie_secure,
         max_age=settings.session_duration_minutes * 60,
     )
-    return LoginResponse(token=signed_token, expires_at=session.expires_at)
+    return LoginResponse(token=signed_token, expires_at=expires_at)
 
 
 @router.post(