Replace deprecated datetime.utcnow with shared naive-UTC helper

All 29 call sites now use app.timeutil.utcnow(), which returns naive
UTC via the non-deprecated API. Semantics are unchanged on purpose:
every datetime column is TIMESTAMP WITHOUT TIME ZONE, so going aware
here would poison naive/aware comparisons. The TIMESTAMPTZ migration
(Phase 2) now has a single place to change. (BE-01)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/app/api/v1/endpoints/accounts.py

@@ -5,6 +5,7 @@ from sqlmodel import Session, select
 
 from app.auth import get_current_user
 from app.db import get_session
+from app.timeutil import utcnow
 from app.models.models import Account, AccountCreate, AccountRead, AccountUpdate
 
 router = APIRouter(prefix="/accounts", tags=["accounts"])
@@ -44,7 +45,7 @@ def update_account(
     update_data = data.model_dump(exclude_unset=True)
     for key, value in update_data.items():
         setattr(account, key, value)
-    account.updated_at = datetime.utcnow()
+    account.updated_at = utcnow()
     session.add(account)
     session.commit()
     session.refresh(account)