feat(serve): relay service prompts from download jobs to the REST client

A job's input_prompt carries a service's request_input() prompt; the client answers it with POST /api/download/jobs/{job_id}/input.

docs/dev/rest-api/job-lifecycle.md

@@ -162,6 +162,7 @@ full detail:
 | `segments_total` | number | Total segments for the track in progress. |
 | `speed` | string | Transfer speed of the track in progress, e.g. `4.20 MB/s`. |
 | `skipped_subtitles` | list | Subtitles that were skipped as non-fatal failures (when `skip_subtitle_errors` is set). |
+| `input_prompt` | string or `null` | The prompt the service waits on. Answer it with `POST /api/download/jobs/{job_id}/input`. |
 
 The full detail adds `started_time`, `completed_time`, `output_files`, the redacted
 `parameters`, and, for failed jobs, `error_message`, `error_details`, `error_code`,
@@ -364,6 +365,10 @@ Three temporary files bridge the parent and the job worker:
 - **`progress.json`**: the job worker writes it continuously as the download proceeds.
 - **`result.json`**: the job worker writes it at the end.
 
+The job worker has no terminal. When a service asks the user for input, the job worker writes
+the prompt to `progress.json` as `input_prompt` and waits. The answer from
+`POST /api/download/jobs/{job_id}/input` goes to the job worker on its stdin.
+
 Running each download out-of-process isolates the server from crashes, memory growth,
 and blocking work inside the download engine, and makes hard cancellation (killing the
 process) reliable.