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/pagination.rs

@@ -34,4 +34,18 @@ impl<T> PagedResponse<T> {
             page: PageInfo { limit, offset, total: Some(total) },
         }
     }
+
+    /// For handlers that compute `total` only on some pages (e.g. the first)
+    /// and leave it `None` elsewhere.
+    pub fn with_optional_total(
+        items: Vec<T>,
+        limit: i64,
+        offset: i64,
+        total: Option<i64>,
+    ) -> Self {
+        Self {
+            items,
+            page: PageInfo { limit, offset, total },
+        }
+    }
 }