feat(webhooks): add delivery attempt history API

[martsokha]

Context

The webhook crate supports retries with exponential backoff, but there is no API for viewing delivery attempts and failures per webhook.

Requirements

• GET /webhooks/{webhookId}/deliveries/ endpoint with cursor pagination
• Each delivery record includes: event type, HTTP status, response body (truncated), timestamp, attempt number
• GET /webhooks/{webhookId}/deliveries/{deliveryId}/ for individual delivery detail
• Retry a failed delivery: POST /webhooks/{webhookId}/deliveries/{deliveryId}/retry

References

• Server docs: ARCHITECTURE.md - Data Model