Add retry logic with exponential backoff to webhookDeliveryQueue

[Kingsman-99]

Description

src/lib/webhookDeliveryQueue.ts delivers webhook events to configured endpoints but makes no retry attempt when a delivery fails. A single network hiccup permanently drops the event, causing silent data loss for webhook consumers.

Acceptance Criteria

• Failed deliveries are retried up to 3 times with exponential backoff (1s, 2s, 4s delays)
• After 3 failed attempts the event is moved to a dead-letter list accessible via a getFailedDeliveries(): WebhookEvent[] export
• Successful delivery on any retry clears the event from the retry queue
• Unit tests: first attempt succeeds (no retry), second attempt succeeds (one retry), all 3 retries fail (event in dead-letter list)
• Existing tests continue to pass

Context

• Target file: src/lib/webhookDeliveryQueue.ts
• Use setTimeout-based delays; no new dependencies

[samueldoo]

Hello Maintainer, I’d love to take ownership of this issue and submit a high-quality pull request once completed.