feat(backend-sep12): [SEP-12] Implement webhook dispatch on KYC status change

[GideonBature]

Summary

Closes #622.

Dispatches signed outbound webhooks to partner interfaces when a customer’s KYC status changes via the SEP-12 provider callback flow (POST /sep12/webhook).

Changes

• Extend WebhookService with sendKycStatusChanged() and a kyc.status_changed partner payload
• Dispatch partner webhooks from Sep12Controller.handleWebhook after a successful status update
• Skip DB updates and partner dispatch when the provider callback does not change status
• Reuse existing webhook delivery behavior: signed POST, retry policy, and x-anchorpoint-* headers
• Add unit and integration coverage for payload shape, delivery, skip conditions, and controller dispatch

Partner webhook payload

{
  "event": "kyc.status_changed",
  "occurredAt": "2026-03-30T10:05:00.000Z",
  "previousStatus": "PENDING",
  "customer": {
    "id": "kyc-1",
    "userId": "user-1",
    "account": "G...",
    "provider": "mock",
    "providerRef": "mock_abc",
    "status": "ACCEPTED",
    "createdAt": "2026-03-30T10:00:00.000Z",
    "updatedAt": "2026-03-30T10:05:00.000Z"
  }
}

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