fix(history): count distinct new chapter numbers, authoritative on detail

new_chapters_count over-counted when scanlations share a chapter number
((manga_id, number) is non-unique, migration 0013): COUNT(*) tallied rows,
so duplicate-numbered chapters inflated the badge. Switch both the list and
per-manga read-progress queries to COUNT(DISTINCT number), and drop the dead
COALESCE (an empty set counts as 0, and the NULL-number case is handled by
the predicate). Fix the misleading comment.

Expose new_chapters_count on GET /me/read-progress/:manga_id and drive the
detail page's "N new" badge from it, instead of recomputing over the
paginated chapter list — which under-counted series past 50 chapters and
disagreed with the homepage shelf. Read markers stay client-side over the
loaded chapters and are documented as by-number (all we persist).

Adds duplicate-scanlation tests at both endpoints.

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

backend/tests/api_history.rs

@@ -242,6 +242,95 @@ async fn list_new_chapters_count_is_zero_without_a_read_chapter(pool: PgPool) {
     assert_eq!(body["items"][0]["new_chapters_count"], 0);
 }
 
+#[sqlx::test(migrations = "./migrations")]
+async fn list_new_chapters_count_dedupes_same_numbered_scanlations(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;
+    // (manga_id, number) is non-unique — two scanlations of chapter 2.
+    // "New since last read" should count distinct chapter numbers past the
+    // reader (2, 3 → 2), not raw rows (2, 2, 3 → 3).
+    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, 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 get_single_manga_reports_new_chapters_count(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 ch1 = seed_chapter(&h.app, &cookie, manga_id, 1).await;
+    let _ = seed_chapter(&h.app, &cookie, manga_id, 2).await;
+    // Duplicate scanlation of chapter 3 must not inflate the count.
+    let _ = seed_chapter(&h.app, &cookie, manga_id, 3).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(
+            &format!("/api/v1/me/read-progress/{manga_id}"),
+            &cookie,
+        ))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::OK);
+    let body = common::body_json(resp).await;
+    // Distinct numbers past chapter 1: {2, 3} → 2.
+    assert_eq!(body["new_chapters_count"], 2);
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn get_single_manga_new_chapters_count_zero_without_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;
+    // Manga-level progress (no chapter) → position unknown → 0.
+    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(
+            &format!("/api/v1/me/read-progress/{manga_id}"),
+            &cookie,
+        ))
+        .await
+        .unwrap();
+    let body = common::body_json(resp).await;
+    assert_eq!(body["new_chapters_count"], 0);
+}
+
 #[sqlx::test(migrations = "./migrations")]
 async fn get_single_manga_returns_404_when_unread(pool: PgPool) {
     let h = common::harness(pool);