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/savings_accrual.py

@@ -5,6 +5,7 @@ from sqlmodel import Session, col, select
 
 from app.auth import get_current_user
 from app.db import get_session
+from app.timeutil import utcnow
 from app.models.models import (
     SavingsAccrual,
     SavingsAccrualCreate,
@@ -44,7 +45,7 @@ def create_accrual(
             detail=f"Accrual for {data.year}-{data.month:02d} already exists (id={existing.id})",
         )
     accrual = SavingsAccrual.model_validate(data)
-    accrual.applied_at = datetime.utcnow()
+    accrual.applied_at = utcnow()
     session.add(accrual)
     session.commit()
     session.refresh(accrual)