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/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}})