feat: add dead-letter queue to webhook dispatcher

[iyanumajekodunmi756]

Summary

• Added capped exponential backoff with jitter to prevent unbounded retry loops
• Implemented dead-letter queue for permanently failing webhook endpoints
• Extended webhook store with dead-letter queue management methods
• Added comprehensive test suite with 90%+ coverage

Changes

• webhook.types.ts: Added WebhookDeliveryStatus type and DeadLetterEntry interface
• webhook.store.ts: Extended with dead-letter queue management (add, get, list, remove methods)
• webhook.dispatcher.ts: Implemented bounded exponential backoff with jitter and DLQ transition
• webhook.dispatcher.test.ts: Created comprehensive test suite covering all edge cases

Technical Details

• Maximum retries: 5 attempts
• Base delay: 1000ms with exponential growth (2^attempt)
• Jitter: Random 0-25% of exponential delay to prevent thundering herd
• Maximum delay cap: 30000ms (30 seconds)
• Dead-letter entries store: webhook config ID, URL, event, payload, attempts, last error, timestamps

Test Coverage

• Successful webhook delivery (first attempt and after retries)
• HMAC signature generation with and without secrets
• Retry behavior on network errors and non-2xx responses
• Exponential backoff with jitter verification
• Maximum delay capping
• Dead-letter queue transition after max retries
• Error recording in dead-letter entries
• Complete payload preservation
• Multiple webhook dispatching (parallel and partial failures)
• Edge cases (timeouts, various HTTP status codes, empty/complex payloads)

Test plan

• Unit tests for webhook dispatcher (706 lines, comprehensive coverage)
• Dead-letter queue functionality tests
• Backoff calculation and jitter tests
• Error handling and edge case tests
• Integration with WebhookStore tests

Acceptance Criteria

• Retries stop after the configured cap (5) and enter dead_letter
• Backoff uses jitter and is bounded (max 30s delay)
• Dead-letter entries record the final error
• Dead-letter entries are exposed for inspection via WebhookStore
• Test coverage exceeds 90%

Closes #317

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