Rotate the event-signing HMAC secret

Rotate the HMAC secret used to sign events delivered to the partner's queue. Either supply a partner-generated high-entropy secret (≥32 bytes, base64 or hex) or omit it to have the server generate one and return it exactly once. The previous secret remains valid for the grace window so signature verifiers can be rolled without dropping events. **Status: not yet implemented.** Returns `503 service_unavailable` with `Retry-After: 86400`. The outbound event publisher is not live yet — contact your TAIH integration contact.

curl -X POST "https://loading/v1/auth/signing-secret/rotate" \  -H "Content-Type: application/json" \  -d '{}'

{
  "data": {
    "signing_secret_rotated_at": "string",
    "previous_secret_expires_at": "string"
  }
}

{
  "detail": [
    {
      "loc": [
        "string"
      ],
      "msg": "string",
      "type": "string"
    }
  ]
}