Add a delivery-attempt cap and dead-letter handling to the webhook dispatcher

[priscaenoch]

Problem

src/webhooks/dispatcher.ts implements outbound delivery with exponential backoff (backoffMs), HMAC signing (subscription secret), and a retry pass for pending deliveries. What is not evident is a maximum-attempt cap or a terminal state for deliveries that keep failing, so a permanently unreachable endpoint can be retried indefinitely, and there is no test coverage asserting the backoff schedule or the signature.

What needs to be done

• Enforce a maximum number of delivery attempts; move exhausted deliveries to a terminal "dead-letter"/failed state rather than retrying forever.
• Expose the failed deliveries (query or metric) so operators can see them.
• Make the max attempts and backoff base configurable via env.
• Add tests covering: the backoff schedule produced by backoffMs, the HMAC signature header, and the transition to the dead-letter state after the cap.

Files

• src/webhooks/dispatcher.ts
• .env.example

Acceptance deliverables

• Deliveries stop after a configurable maximum and land in a visible terminal state.
• Backoff and signing behavior are covered by tests.
• All CI checks pass; the change cannot be merged until CI is green.

Tests to pass

• Test: backoffMs returns the expected increasing schedule.
• Test: the delivery HMAC signature matches the subscription secret.
• Test: a delivery is dead-lettered after the maximum attempts.

[DEV-DESIGN6]

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