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/budget.py

@@ -7,6 +7,7 @@ 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 (
     BalanceOverride,
     BalanceOverrideCreate,
@@ -259,7 +260,7 @@ def upsert_balance_override(
 
     if existing:
         existing.override_balance = data.override_balance
-        existing.updated_at = datetime.utcnow()
+        existing.updated_at = utcnow()
         session.add(existing)
         session.commit()
         session.refresh(existing)