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/endpoints.md

@@ -551,7 +551,8 @@ Show download jobs, with optional filtering and sorting.
           "segments_done": 120,
           "segments_total": 300,
           "speed": "5.2 MB/s",
-          "skipped_subtitles": []
+          "skipped_subtitles": [],
+          "input_prompt": null
         }
       ]
     }
@@ -709,6 +710,36 @@ Move a queued job to the front of the download queue.
 | `404` | `JOB_NOT_FOUND` | No such job. |
 | `409` | `CONFLICT` | Job is not queued. |
 
+### `POST /api/download/jobs/{job_id}/input`
+
+Answer the prompt that a running job waits on. A service can ask for input during a job, for
+example an OTP code, a PIN, or a device-code confirmation. The job then shows the prompt in its
+`input_prompt` field and in its `progress` events. Show the prompt to the user and send the
+answer here.
+
+=== "Request"
+
+    ```json
+    { "response": "123456" }
+    ```
+
+=== "Response `200`"
+
+    ```json
+    { "status": "accepted" }
+    ```
+
+| Status | Error code | Meaning |
+| --- | --- | --- |
+| `200` | - | Answer sent to the job. The `input_prompt` field goes back to `null`. |
+| `400` | `INVALID_INPUT` | The body is not JSON, or it has no `response`. |
+| `404` | `JOB_NOT_FOUND` | No such job. |
+| `409` | `CONFLICT` | The job does not wait on a prompt. |
+
+!!! warning
+    The job keeps its download slot while it waits. It fails with `AUTH_FAILED` if nobody
+    answers the prompt within 10 minutes.
+
 ---
 
 ## Server management

docs/dev/rest-api/index.md

@@ -166,6 +166,7 @@ request and response in full.
 | `DELETE` | `/api/download/jobs/{job_id}` | Cancel a running job, or remove a finished one. |
 | `POST` | `/api/download/jobs/{job_id}/retry` | Re-queue a finished job's parameters as a new job. |
 | `POST` | `/api/download/jobs/{job_id}/priority` | Move a queued job to the front. |
+| `POST` | `/api/download/jobs/{job_id}/input` | Answer the prompt that a running job waits on. |
 
 ### History and server info
 

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.