feat(backend): add webhook retry scheduler

[brentthomas248]

Closes #1710

Summary:

• Repair webhook queue singleton reset so the backend test suite can reset queue state safely.
• Make retry backoff deterministic and testable, including first-failure scheduling from the first delay bucket.
• Add claimDue(now) so workers claim pending deliveries and failed deliveries only when their retry is due.
• Expose deadLetterCount in webhook queue stats.
• Extend v009 webhook_queue schema with attempts, max_attempts, next_attempt_at, dead_letter status, and a status/next_attempt scheduler index.
• Add focused Jest coverage for retry timing, due claiming, DLQ stats, and migration schema.

Tests:

• npm --prefix backend test -- webhook-delivery-repo.test.ts webhookQueue.persist.test.ts webhook-queue-migration.test.ts --runInBand --coverage=false
• git diff --check

Notes:

• npx tsc --noEmit is currently blocked by a pre-existing syntax error in backend/src/index.ts around lines 78-79, outside this patch.