feat(history): report personal new-chapter count on read-progress list

Add `new_chapters_count` to each `GET /me/read-progress` item: how many
chapters sit past the reader's last-read chapter, computed by a correlated
subquery on chapter number. It is 0 when the reader is caught up, and 0
when the last-read chapter is unknown (manga-level progress or a deleted
chapter) so we never over-claim. (manga_id, number) is non-unique, so this
counts positions past the reader rather than distinct files.

Backs the personal "new since last read" indicator on the upcoming
Continue-reading shelf. Tests cover the mid-series, caught-up, and
no-read-chapter cases.

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

backend/tests/api_history.rs

@@ -164,6 +164,84 @@ async fn list_is_per_user_only(pool: PgPool) {
     assert_eq!(body["items"], json!([]));
 }
 
+#[sqlx::test(migrations = "./migrations")]
+async fn list_reports_new_chapters_since_last_read(pool: PgPool) {
+    let h = common::harness(pool);
+    let (_, cookie) = common::register_user(&h.app).await;
+    let manga_id = common::seed_manga_via_api(&h.app, &cookie, "Berserk").await;
+    // Three chapters exist; the reader is on chapter 1, so chapters 2 and
+    // 3 are "new since last read".
+    let ch1 = seed_chapter(&h.app, &cookie, manga_id, 1).await;
+    let _ = seed_chapter(&h.app, &cookie, manga_id, 2).await;
+    let _ = seed_chapter(&h.app, &cookie, manga_id, 3).await;
+    let _ = upsert_progress(
+        &h.app,
+        &cookie,
+        json!({ "manga_id": manga_id.to_string(), "chapter_id": ch1, "page": 1 }),
+    )
+    .await;
+
+    let resp = h
+        .app
+        .clone()
+        .oneshot(common::get_with_cookie("/api/v1/me/read-progress", &cookie))
+        .await
+        .unwrap();
+    let body = common::body_json(resp).await;
+    assert_eq!(body["items"][0]["new_chapters_count"], 2);
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn list_new_chapters_count_is_zero_at_latest_chapter(pool: PgPool) {
+    let h = common::harness(pool);
+    let (_, cookie) = common::register_user(&h.app).await;
+    let manga_id = common::seed_manga_via_api(&h.app, &cookie, "Berserk").await;
+    let _ = seed_chapter(&h.app, &cookie, manga_id, 1).await;
+    let ch2 = seed_chapter(&h.app, &cookie, manga_id, 2).await;
+    // Caught up to the last chapter → nothing newer.
+    let _ = upsert_progress(
+        &h.app,
+        &cookie,
+        json!({ "manga_id": manga_id.to_string(), "chapter_id": ch2, "page": 1 }),
+    )
+    .await;
+
+    let resp = h
+        .app
+        .clone()
+        .oneshot(common::get_with_cookie("/api/v1/me/read-progress", &cookie))
+        .await
+        .unwrap();
+    let body = common::body_json(resp).await;
+    assert_eq!(body["items"][0]["new_chapters_count"], 0);
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn list_new_chapters_count_is_zero_without_a_read_chapter(pool: PgPool) {
+    let h = common::harness(pool);
+    let (_, cookie) = common::register_user(&h.app).await;
+    let manga_id = common::seed_manga_via_api(&h.app, &cookie, "Berserk").await;
+    let _ = seed_chapter(&h.app, &cookie, manga_id, 1).await;
+    let _ = seed_chapter(&h.app, &cookie, manga_id, 2).await;
+    // Progress recorded without a chapter (manga-level) — we can't place
+    // the reader among the chapters, so we don't claim any are "new".
+    let _ = upsert_progress(
+        &h.app,
+        &cookie,
+        json!({ "manga_id": manga_id.to_string(), "page": 1 }),
+    )
+    .await;
+
+    let resp = h
+        .app
+        .clone()
+        .oneshot(common::get_with_cookie("/api/v1/me/read-progress", &cookie))
+        .await
+        .unwrap();
+    let body = common::body_json(resp).await;
+    assert_eq!(body["items"][0]["new_chapters_count"], 0);
+}
+
 #[sqlx::test(migrations = "./migrations")]
 async fn get_single_manga_returns_404_when_unread(pool: PgPool) {
     let h = common::harness(pool);