feat(backend): Build webhook delivery system with retry logic

[KuchiMercy]

Description

Webhook module has entity/controller stubs. Build complete webhook registration, delivery, and management system.

Existing: webhook.entity.ts, webhook.controller.ts, webhook.module.ts

Acceptance Criteria

• Webhook entity: id, userId, url, secret, events, isActive, lastTriggeredAt, failureCount
• WebhookDelivery entity: id, webhookId, event, payload, responseStatus, attemptCount, nextRetryAt
• POST /webhooks (register with auto-generated signing secret)
• GET /webhooks, PATCH /webhooks/:id, DELETE /webhooks/:id
• GET /webhooks/:id/deliveries, POST retry
• HTTP POST delivery with HMAC-SHA256 signing (X-Vaultix-Signature header)
• Retry: 1min, 5min, 30min, 2h, 12h (max 5 attempts)
• After 5 failures: mark inactive, notify user
• URL validation: HTTPS required in production, test ping
• POST /webhooks/:id/test for verification
• Max 10 webhooks per user
• Unit and integration tests

Points: 200

[amina69]

@amina69 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

hi, let me work on this

ℹ️ Repo Maintainers: To accept this application, review their application or assign @amina69 to this issue.

[chinweobtagaz]

@chinweobtagaz has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

hi maintainer, can i work on this issue ??

ℹ️ Repo Maintainers: To accept this application, review their application or assign @chinweobtagaz to this issue.

[drips-wave]

Congratulations, @amina69! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @amina69: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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