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/tests/api_mangas.rs

@@ -99,6 +99,42 @@ async fn list_returns_total_count_independent_of_pagination(pool: PgPool) {
     assert_eq!(body["page"]["total"], 3);
 }
 
+#[sqlx::test(migrations = "./migrations")]
+async fn list_total_is_computed_only_on_the_first_page(pool: PgPool) {
+    let h = common::harness(pool);
+    let (_, cookie) = common::register_user(&h.app).await;
+    for title in ["One Piece", "Berserk", "Vinland Saga"] {
+        seed(&h.app, &cookie, title).await;
+    }
+
+    // Page 1 (offset 0): total is the full population.
+    let body0 = common::body_json(
+        h.app
+            .clone()
+            .oneshot(common::get("/api/v1/mangas?limit=2&offset=0"))
+            .await
+            .unwrap(),
+    )
+    .await;
+    assert_eq!(body0["page"]["total"], 3);
+
+    // Page 2 (offset 2): total is omitted (null) — the correlated count is
+    // not recomputed on every page. Items still paginate correctly.
+    let body1 = common::body_json(
+        h.app
+            .oneshot(common::get("/api/v1/mangas?limit=2&offset=2"))
+            .await
+            .unwrap(),
+    )
+    .await;
+    assert!(
+        body1["page"]["total"].is_null(),
+        "total should be null past the first page, got {}",
+        body1["page"]["total"]
+    );
+    assert_eq!(body1["items"].as_array().unwrap().len(), 1);
+}
+
 #[sqlx::test(migrations = "./migrations")]
 async fn search_via_trigram_tolerates_typos(pool: PgPool) {
     let h = common::harness(pool);