#54 FTX-54: Durable webhook outbox + drainer + subscriptions CRUD

enqueue_webhook writes webhook_deliveries in caller transaction (replaces fire-and-forget asyncio.create_task); drainer with HMAC (UB-WH-2), backoff 10s-8h via next_attempt_at, DLQ state=dead + audit (EV-JOB-1); /v1/webhooks CRUD (UB-WH-1) with Fernet-encrypted secrets (UB-SEC-6); per-decision webhook_url column honored (UB-WH-6). Tests: transactionality, retry schedule, DLQ, signature verify (AC-10). Phase 3.