refactor: make POST /retry take a singular id

The endpoint accepted {ids: [x]} and then rejected anything but exactly one
id, so the schema advertised a batch it never supported. Retry is genuinely
singular: unlike the /delete, /start and /cancel batches, which act on local
state and can't meaningfully fail for one id and not another, each retry
re-extracts the URL and the caller removes that item's done record only once
it is confirmed re-queued. A real batch form would need per-id results in the
response for the caller to know which records to remove; that only becomes
worth designing alongside moving done-record deletion server-side.

/retry has not shipped yet, so there is no compatibility cost.

app/main.py

@@ -895,11 +895,12 @@ async def cancel_add(request):
 
 @routes.post(config.URL_PREFIX + 'retry')
 async def retry(request):
+    # Singular by design, unlike the 'ids' batch endpoints: a retry re-extracts
+    # the URL, so it can fail per item, and the caller removes that item's done
+    # record only once it is confirmed re-queued. A batch form would have to
+    # report per-id results for the caller to know which ones to remove.
     post = await _read_json_request(request)
-    ids = _require_id_list(post)
-    if len(ids) != 1:
-        raise web.HTTPBadRequest(reason="'ids' must contain exactly one download id")
-    status = await dqueue.retry(ids[0])
+    status = await dqueue.retry(_require_id(post))
     return web.Response(text=serializer.encode(status), content_type='application/json')
 
 
@@ -995,6 +996,13 @@ async def subscriptions_check(request):
     result = await submgr.check_now([str(i) for i in ids] if ids else None)
     return web.Response(text=serializer.encode(result))
 
+def _require_id(post: dict) -> str:
+    id = post.get('id')
+    if not isinstance(id, str) or not id:
+        raise web.HTTPBadRequest(reason="'id' must be a non-empty string")
+    return id
+
+
 def _require_id_list(post: dict) -> list:
     ids = post.get('ids')
     if not isinstance(ids, list) or not ids or not all(isinstance(i, str) for i in ids):

app/tests/test_api.py

@@ -72,12 +72,20 @@ async def test_add_ok(mock_dqueue):
 
 @pytest.mark.asyncio
 async def test_retry_passes_failed_download_id(mock_dqueue):
-    req = _json_request({"ids": ["https://example.com/watch?v=1"]})
+    req = _json_request({"id": "https://example.com/watch?v=1"})
     resp = await main.retry(req)
     assert resp.status == 200
     mock_dqueue.retry.assert_awaited_once_with("https://example.com/watch?v=1")
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize("body", [{}, {"id": ""}, {"id": ["a"]}, {"ids": ["a"]}])
+async def test_retry_rejects_missing_or_non_string_id(mock_dqueue, body):
+    with pytest.raises(web.HTTPBadRequest):
+        await main.retry(_json_request(body))
+    mock_dqueue.retry.assert_not_awaited()
+
+
 @pytest.mark.asyncio
 async def test_add_passes_preset_and_overrides(mock_dqueue, monkeypatch):
     monkeypatch.setattr(main.config, "YTDL_OPTIONS_PRESETS", {"Preset A": {"writesubtitles": True}})

ui/src/app/services/downloads.service.spec.ts

@@ -121,7 +121,7 @@ describe('DownloadsService', () => {
     service.retry('https://example.com/v').subscribe();
     const req = httpMock.expectOne('retry');
     expect(req.request.method).toBe('POST');
-    expect(req.request.body).toEqual({ ids: ['https://example.com/v'] });
+    expect(req.request.body).toEqual({ id: 'https://example.com/v' });
     req.flush({ status: 'ok' });
   });
 

ui/src/app/services/downloads.service.ts

@@ -170,7 +170,7 @@ export class DownloadsService {
   }
 
   public retry(id: string) {
-    return this.http.post<Status>('retry', { ids: [id] }).pipe(
+    return this.http.post<Status>('retry', { id: id }).pipe(
       catchError(this.handleHTTPError)
     );
   }