RabbitMQ durable job queue for email outbox, webhooks, and AI tasks

Context: The email outbox drains inline in the request path, and outbound Slack/Discord webhooks fire synchronously. Under a traffic spike this adds latency and risks lost work.

Scope:

• Stand up a RabbitMQ producer/consumer with ack, retry, and a dead-letter queue.
• Move email_outbox processing to a worker.
• Move outbound webhook delivery to the queue with retries and idempotency (webhook_events).
• Reserve a queue for future AI tasks (drafts, moderation).

Infra: RabbitMQ is already available on the LAN. Add the AMQP URL to /opt/hollahoop/.env.

Done when: emails and webhooks are enqueued and delivered by a worker, with retries and visibility into failures.