fix: compute the /mangas total only on the first page

GET /mangas recomputed count(*) over FILTER_WHERE (up to five correlated
subqueries) on every page, so pagination cost scaled with catalog size. The
total doesn't change as a caller walks pages, so compute it once at offset 0
and return null thereafter (the envelope already serialises total as
Option<i64>; the frontend types it number|null and the library route ignores
it).

Bump to 0.124.7.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/api/mangas.rs

@@ -157,7 +157,9 @@ async fn list(
         order,
     };
     let (items, total) = repo::manga::list_cards(&state.db, &q).await?;
-    Ok(Json(PagedResponse::with_total(items, limit, offset, total)))
+    Ok(Json(PagedResponse::with_optional_total(
+        items, limit, offset, total,
+    )))
 }
 
 async fn get_one(