check_usage now checks the billable user's quota on every request, before
the per-agent 24h limits, which keep applying to traffic through an agent.
Until now a request without an agent key skipped every limit. A refusal is
a 429 with Retry-After and a body naming the budget, usage, limit, the
layer the limit came from and when it resets.
Headless runs check the agent owner's quota before starting. A refused
scheduled run is recorded as budget_exceeded; a refused webhook run returns
a quota_exceeded result instead of raising, so Celery does not retry it.
Alex
5406550bca