Add webhook notifications for new blocks

[Bortlesboat]

Summary

Allow users to register webhook URLs that get called when specific events occur (new block, large mempool tx, fee spike).

Proposed API

POST /api/v1/webhooks
{
  "url": "https://myapp.com/callback",
  "events": ["new_block", "fee_spike"],
  "secret": "optional-hmac-secret"
}

Events

• new_block — fires on each new block with height, hash, tx count
• fee_spike — fires when recommended fee jumps >50% in 10 minutes
• large_tx — fires for mempool txs above configurable size threshold

Acceptance criteria

• Webhook registration endpoint (authed)
• Background worker that checks events and POSTs to registered URLs
• HMAC signature on webhook payload for verification
• Retry logic (3 attempts with backoff)
• Tests