Get Itsm Credentials Internal

Get ITSM credentials - internal use only (no auth). Story #214: Config drift fix - returns actual credentials from database. Used by LiveKit Agent MCP subprocess to get DB-configured credentials. WARNING: Returns actual credentials. Only accessible from K8s cluster. Requires X-Internal-Secret header (fail-closed: rejects if not configured). Issue #398 S4: changed from fail-open to fail-closed.

curl -X GET "https://loading/api/internal/configuration/itsm-credentials"

{}

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