Start/stop always-on alert rules for an incoming camera event

Request body for POST /api/v1/realtime/always-on.

Only the canonical VST shape is accepted:

{
  "source": "vst",
  "alert_type": "camera_status_change",
  "created_at": "2026-04-22T17:38:38Z",
  "event": {
    "camera_id": "c0413489-6ca1-422e-a09c-08224169ff6a",
    "camera_name": "warehouse",
    "camera_url": "rtsp://localhost:8554/live/<id>",
    "camera_vod_url": "rtsp://localhost:8554/vod/<id>",
    "change": "camera_streaming",
    "metadata": {"codec": "H264"}
  }
}

Producer-only fields on the outer envelope (source, alert_type, created_at) and inside event (metadata, camera_vod_url) pass through unused; extra unknown fields are tolerated (extra="allow") so upstream changes don't break the endpoint.