All 4 webhook controllers now verify the signature, insert an Oban job,
and return immediately. Previously, Stripe and PagerDuty did DB queries +
writes inline; Gaiia ran upserts; Agent Release broadcast to all connected
WebSockets — all in the request path.
New workers:
- StripeWebhookWorker — delegates to WebhookProcessor.process/1
- GaiiaWebhookWorker — delegates to Webhooks.process_event/3
- PagerdutyWebhookWorker — resolve/acknowledge alerts by dedup key
- AgentReleaseWebhookWorker — broadcast mass update to agents
Also fix: log error when resolve_alert fails in agent_channel (was silently
discarded, causing "Resolving stuck device_down alert" to repeat in logs).
