on_error

          - { name: title, value: "Rehearsal item" }   # SLOT: the metadata fields
        mode: jq
        jq: "{ id: .id, url: .url }"   # SLOT: the fields downstream needs
    on_error:
      # The rehearsal, NARROWED to the one thing it may forgive: the key is
      # not configured. A `secrets:` entry whose env var is unset fails
      # NIKA-VAR-001 before any byte leaves (measured on 0.118.7 · the run
      # settles « 1 recovered » on that code) — that absence IS the rehearsal
      # switch. Everything else stays loud: a wired key the API rejects is
      # NIKA-BUILTIN-FETCH-001 naming the 401 (measured), a dead endpoint too.