feat: retry engine with exponential back-off for failed deliveries

[cokehill]

Summary

Failed deliveries must be automatically retried with exponential back-off so transient
outages (receiver down, SMTP blip) do not cause permanent notification loss.

Acceptance criteria

• Max retries configurable via WEBHOOK_MAX_RETRIES (default 5)
• Base delay configurable via WEBHOOK_RETRY_BASE_MS (default 1000ms)
• Delay formula: BASE * 2^(attempt - 1) ms
• After max retries, delivery status set to failed in DB
• POST /api/deliveries/:id/retry resets attempts = 0 and status = pending
• Unit test: assert 3 failures → 3 delays with correct multipliers
• Unit test: assert 5 failures → status = failed

Notes

Back-off runs inside DeliveryService.dispatch() — no separate cron job needed for v0.1.