feat: list + get chapters under /api/v1/mangas/{id}/chapters

Both endpoints are public reads — anyone browsing can see a manga's
table of contents and chapter metadata. Uploads land in feat/uploads.

- GET /api/v1/mangas/{id}/chapters returns the paged envelope
  ({items, page}) ordered by chapter number ASC. Surfaces 404 if the
  parent manga doesn't exist so an empty result can't be mistaken for
  "no chapters yet" on a real manga.
- GET /api/v1/mangas/{id}/chapters/{number} returns a single chapter,
  404 if either manga or chapter is missing.

repo::chapter exposes list_for_manga, find_by_manga_and_number, and
create. create translates the (manga_id, number) unique violation into
AppError::Conflict so the upload handler can later return a clean 409.

Frontend lib/api/chapters.ts mirrors the shape with listChapters and
getChapter; Vitest asserts the URL shape, paged response handling, and
404 envelope propagation.

Lockstep version bump to 0.4.0.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/tests/api_chapters.rs

@@ -0,0 +1,144 @@
+mod common;
+
+use axum::http::StatusCode;
+use serde_json::json;
+use sqlx::PgPool;
+use tower::ServiceExt;
+use uuid::Uuid;
+
+/// Create a manga via the API (which requires auth) and return its id +
+/// the session cookie of the user who owns it.
+async fn seed_manga(h: &common::Harness, cookie: &str, title: &str) -> Uuid {
+    let resp = h
+        .app
+        .clone()
+        .oneshot(common::post_json_with_cookie(
+            "/api/v1/mangas",
+            json!({ "title": title }),
+            cookie,
+        ))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::OK);
+    let body = common::body_json(resp).await;
+    Uuid::parse_str(body["id"].as_str().unwrap()).unwrap()
+}
+
+/// Insert a chapter directly via the repo (the upload handler that does
+/// this from HTTP lands in feat/uploads).
+async fn seed_chapter(pool: &PgPool, manga_id: Uuid, number: i32, title: Option<&str>) {
+    mangalord::repo::chapter::create(pool, manga_id, number, title)
+        .await
+        .unwrap();
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn list_chapters_is_empty_initially(pool: PgPool) {
+    let h = common::harness(pool);
+    let (_, cookie) = common::register_user(&h.app).await;
+    let manga_id = seed_manga(&h, &cookie, "Berserk").await;
+
+    let resp = h
+        .app
+        .oneshot(common::get(&format!("/api/v1/mangas/{manga_id}/chapters")))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::OK);
+    let body = common::body_json(resp).await;
+    assert_eq!(body["items"], json!([]));
+    assert_eq!(body["page"]["limit"], 50);
+    assert_eq!(body["page"]["offset"], 0);
+    assert!(body["page"]["total"].is_null());
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn list_chapters_returned_in_number_order(pool: PgPool) {
+    let h = common::harness(pool.clone());
+    let (_, cookie) = common::register_user(&h.app).await;
+    let manga_id = seed_manga(&h, &cookie, "Berserk").await;
+
+    seed_chapter(&pool, manga_id, 3, Some("The Black Swordsman")).await;
+    seed_chapter(&pool, manga_id, 1, Some("The Brand")).await;
+    seed_chapter(&pool, manga_id, 2, None).await;
+
+    let resp = h
+        .app
+        .oneshot(common::get(&format!("/api/v1/mangas/{manga_id}/chapters")))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::OK);
+    let body = common::body_json(resp).await;
+    let numbers: Vec<i64> = body["items"]
+        .as_array()
+        .unwrap()
+        .iter()
+        .map(|c| c["number"].as_i64().unwrap())
+        .collect();
+    assert_eq!(numbers, vec![1, 2, 3]);
+    assert_eq!(body["items"][1]["title"], serde_json::Value::Null);
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn list_chapters_returns_404_for_unknown_manga(pool: PgPool) {
+    let h = common::harness(pool);
+    let unknown = Uuid::nil();
+    let resp = h
+        .app
+        .oneshot(common::get(&format!("/api/v1/mangas/{unknown}/chapters")))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::NOT_FOUND);
+    let body = common::body_json(resp).await;
+    assert_eq!(body["error"]["code"], "not_found");
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn get_chapter_by_number(pool: PgPool) {
+    let h = common::harness(pool.clone());
+    let (_, cookie) = common::register_user(&h.app).await;
+    let manga_id = seed_manga(&h, &cookie, "Berserk").await;
+    seed_chapter(&pool, manga_id, 1, Some("The Brand")).await;
+
+    let resp = h
+        .app
+        .oneshot(common::get(&format!(
+            "/api/v1/mangas/{manga_id}/chapters/1"
+        )))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::OK);
+    let body = common::body_json(resp).await;
+    assert_eq!(body["number"], 1);
+    assert_eq!(body["title"], "The Brand");
+    assert_eq!(body["page_count"], 0);
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn get_chapter_unknown_number_is_404(pool: PgPool) {
+    let h = common::harness(pool);
+    let (_, cookie) = common::register_user(&h.app).await;
+    let manga_id = seed_manga(&h, &cookie, "Berserk").await;
+
+    let resp = h
+        .app
+        .oneshot(common::get(&format!(
+            "/api/v1/mangas/{manga_id}/chapters/99"
+        )))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::NOT_FOUND);
+    let body = common::body_json(resp).await;
+    assert_eq!(body["error"]["code"], "not_found");
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn get_chapter_unknown_manga_is_404(pool: PgPool) {
+    let h = common::harness(pool);
+    let unknown = Uuid::nil();
+    let resp = h
+        .app
+        .oneshot(common::get(&format!("/api/v1/mangas/{unknown}/chapters/1")))
+        .await
+        .unwrap();
+    assert_eq!(resp.status(), StatusCode::NOT_FOUND);
+}