[huangji6693-max]

Summary

Adds deterministic unit test coverage in backend/src/__tests__/webhookService.test.ts for the webhook subscription lifecycle and delivery filtering in backend/src/services/webhookService.ts:

• Subscription lifecycle: Covers registerSubscription (mapping callbackUrl, eventTypes, isActive, createdAt, updatedAt), listSubscriptions, and idempotent deleteSubscription (true on deletion, false when already deleted).
• Event filtering: Verifies WebhookService.isSupported against all SUPPORTED_WEBHOOK_EVENT_TYPES and confirms dispatch filters out unsubscribed event types via event_types @> $1::jsonb.
• Deletion during active delivery: Verifies that concurrent deletion of one subscription mid-flight (causing a foreign-key rejection when logging its delivery) is isolated and does not abort webhook delivery or persistence for remaining active subscribers.
• Secret rotation: Verifies rotating a subscription's HMAC secret (secret-v1 -> secret-v2 -> null), asserting that x-remitlend-signature (t=<timestamp>,v1=<hmac_sha256>) updates to match the new secret, invalidates signatures under the previous secret, and is omitted once the secret is cleared.

Closes #2079

Verification

• node --experimental-vm-modules node_modules/jest/bin/jest.js src/__tests__/webhookService.test.ts: 17 passed, 0 failed.

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