feat(backend): add GET /api/webhooks endpoint listing all registered low-balance webhook URLs

[Dev-AdeTutu]

Summary

Once a provider registers a webhook via POST /api/webhooks/low-balance there is no way to see which URLs are registered. Add a read endpoint so admins can audit and debug webhook configuration.

Acceptance Criteria

• GET /api/webhooks returns { webhooks: [{ url, registered_at }] }
• Protected by requireAdminKey middleware
• Returns an empty array when no webhooks are registered

Files to touch

• backend/src/routes/webhooks.ts
• backend/src/lib/webhookRegistry.ts — add a listWebhooks() export

[ironhood]

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

Hi Maintainer, can I work on this ? I will create quality PR per your requirement. looking forward to contributing

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