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.

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)
     );
   }