Third-party API quota exhausted

Your delivery app's address-autocomplete and route features go partly dark at 14:20. Dashboards: calls to the Google Maps Platform API start returning HTTP 403 with 'OVER_QUERY_LIMIT' / quota exceeded; the affected features error at ~70%; your own infra is healthy. Recent context: marketing ran a flash promo this morning that ~doubled active users, and last week an engineer removed a client-side autocomplete debounce to 'make typing feel snappier.' Daily quota is a fixed per-project cap. How do you triage and mitigate?