Webhook registration and dispatch on payment

[iexwr]

Context

External services (shops, bots) want to be notified when a request is paid, instead of polling.

Acceptance criteria

• POST /api/v1/webhooks accepts { url, secret }, stores a Webhook row, returns its id.
• When the indexer records a request_paid, enqueue a BullMQ job that POSTs { requestId, payer, status } (with secret header) to every registered webhook.
• Failed deliveries retry with backoff (BullMQ defaults acceptable).

Technical notes

• Validate url with Zod (z.string().url()).
• Use the existing ioredis/bullmq deps; create a webhooks queue.

Testing

• Test that registering a webhook and simulating a paid event results in a POST to the registered URL (local test server / nock).