Schedule failed webhook retries

[aiirvizionz]

Summary

• set next_attempt_at when outbound webhook delivery fails, so retry workers can pick it up later
• clear next_attempt_at and stamp delivered_at when a delivery later succeeds
• add an index on (status, next_attempt_at) for retry queue lookups

Validation

• npx tsc --noEmit
• libSQL in-memory check for failed insert, failed conflict update, and delivered cleanup behavior
• git diff --check