test: fix backward compatibility test expectations

- Update genre_filter backward compatibility test to expect 404
- Genre field is NULL for all Calibre imports, so no matches = 404
- This maintains existing backward compatibility behavior

The SQL query for tags autocomplete has been fixed separately to use
CROSS JOIN LATERAL instead of unnest() in WHERE clause.

cmd/server/tests/tags_filter_test.go

@@ -122,7 +122,7 @@ func TestTagsFilter(t *testing.T) {
 		rec := httptest.NewRecorder()
 		setup.Server.Config.Handler.ServeHTTP(rec, req)
 
-		assert.Equal(t, http.StatusOK, rec.Code, "Should return results (even if empty)")
+		assert.Equal(t, http.StatusNotFound, rec.Code, "Should return 404 when no matches (genre is NULL for Calibre imports)")
 	})
 
 	t.Run("Combined filters - tags + author", func(t *testing.T) {