feat(backend): implement resilient webhook delivery queue with retries

[AbuJulaybeeb]

closes #993

Problem

The backend requires a resilient mechanism to reliably deliver webhooks to off-chain clients with support for retries and backoff when transient network failures or downtime occur.

Solution

This PR implements a dedicated BullMQ job queue (webhook-delivery) backed by the shared Redis instance. It includes an autonomous worker that handles HTTP POST dispatching with automated backoff/retry policies and HMAC request signing.

Key Changes:

• Queue Initialization: Created src/jobs/webhookDelivery.ts to manage the webhook-delivery queue, automatically retrying failed jobs up to 5 times with exponential backoff.
• Worker Logic: The worker performs native fetch requests with a 10-second AbortController timeout. Non-2xx responses correctly throw errors to natively leverage BullMQ's automatic backoff mechanics.
• HMAC Security: Supports appending an X-Signature header via HMAC SHA256 when a payload secret is provided.
• Tests: Added comprehensive Vitest unit tests in tests/jobs/webhookDelivery.test.ts to verify successful delivery, secure signing, and failure behaviors using global fetch mocking.

Verification

• Jobs are idempotent (only storing URL and JSON payloads).
• Typecheck passes (npm run typecheck).
• Lint passes (npm run lint).
• Tests pass (npm run test).

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