List import jobs

StageProgressAvailable tells the client that GET /imports/{id}/stage-progress will answer for this business (the model_extraction dev feature is on), so the per-stage dropdown can be rendered and fetched lazily. It rides on a response the page already loads, which is why an unflagged tenant issues no extra request to discover it. omitempty keeps the unflagged payload byte-identical to the pre-stage-progress response.