Add webhook push notifications for transaction and agent events

[robertocarlous]

Summary

Clients (mobile apps, dashboards) currently have no server-push mechanism for transaction events. A webhook system would let integrators subscribe to events like transaction.confirmed, agent.rebalanced, deposit.received.

Proposed Solution

• Add WebhookSubscription Prisma model: url, events (array), secret (for HMAC-SHA256 signature), userId
• Add CRUD endpoints under /api/webhooks
• Dispatch webhooks from event listener and agent loop post-action
• Sign payload with X-Neurowealth-Signature header
• Retry failed deliveries up to 3 times with exponential backoff (leverage existing fetchWithRetry)

Acceptance Criteria

• POST /api/webhooks creates subscription, returns signing secret once
• Payload signed with HMAC-SHA256, verifiable by recipient
• Failed deliveries retried and logged in a WebhookDelivery table
• Unit tests for signature generation and retry logic
• OpenAPI spec updated

[GreatShinro]

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

Hi, can I work on this please?

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

[drips-wave]

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

🧑‍💻 @GreatShinro: 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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