fix(lessons): deleteLesson matches apiDelete's contract (#3734)

CI 7059's typecheck, two errors on one line: `apiDelete` returns
`Promise<void>` and takes no type argument. I had given it the response body's
shape, which it discards.

Matched to how snippets delete, rather than adding a second delete helper to
carry the batch id — no caller has wanted it, and the second helper would be
the duplication rather than the feature.

Everything else in the UI batch typechecked clean.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

frontend/src/api/lessons.ts

@@ -124,12 +124,12 @@ export function updateLesson(
   return apiPatch<Lesson>(`/api/lessons/${id}`, payload);
 }
 
-export function deleteLesson(
-  id: number,
-): Promise<{ deleted: number; deleted_batch_id: string }> {
-  return apiDelete<{ deleted: number; deleted_batch_id: string }>(
-    `/api/lessons/${id}`,
-  );
+/** Trash, not erase — recoverable. `apiDelete` discards the body, which is the
+ *  established shape here (snippets delete the same way): the batch id is in
+ *  the response, but no caller has needed it and inventing a second delete
+ *  helper to carry it would be the duplication, not the feature. */
+export function deleteLesson(id: number): Promise<void> {
+  return apiDelete(`/api/lessons/${id}`);
 }
 
 /** The lessons drawn FROM one record — the reverse of `learned_from`.