feat(quotas): enforce user quotas on chat, agent, scheduled and webhook runs

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.

docsgpt/api/v1/routes.py

@@ -338,7 +338,7 @@ def chat_completions():
             )
 
         helper = _V1AnswerHelper()
-        usage_error = helper.check_usage(processor.agent_config)
+        usage_error = helper.check_usage(processor.agent_config, processor.decoded_token)
         if usage_error:
             return usage_error