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/src/repo/chapter.rs

@@ -0,0 +1,88 @@
+//! Chapter persistence.
+
+use sqlx::PgPool;
+use uuid::Uuid;
+
+use crate::domain::Chapter;
+use crate::error::AppResult;
+
+pub async fn list_for_manga(
+    pool: &PgPool,
+    manga_id: Uuid,
+    limit: i64,
+    offset: i64,
+) -> AppResult<Vec<Chapter>> {
+    let rows = sqlx::query_as::<_, Chapter>(
+        r#"
+        SELECT id, manga_id, number, title, page_count, created_at
+        FROM chapters
+        WHERE manga_id = $1
+        ORDER BY number ASC
+        LIMIT $2 OFFSET $3
+        "#,
+    )
+    .bind(manga_id)
+    .bind(limit)
+    .bind(offset)
+    .fetch_all(pool)
+    .await?;
+    Ok(rows)
+}
+
+pub async fn find_by_manga_and_number(
+    pool: &PgPool,
+    manga_id: Uuid,
+    number: i32,
+) -> AppResult<Option<Chapter>> {
+    let row = sqlx::query_as::<_, Chapter>(
+        r#"
+        SELECT id, manga_id, number, title, page_count, created_at
+        FROM chapters
+        WHERE manga_id = $1 AND number = $2
+        "#,
+    )
+    .bind(manga_id)
+    .bind(number)
+    .fetch_optional(pool)
+    .await?;
+    Ok(row)
+}
+
+/// Inserts a chapter. Used by tests today and by the upload handler in
+/// feat/uploads. Returns `AppError::Conflict` on the (manga_id, number)
+/// unique violation so handlers can surface a clean 409.
+pub async fn create(
+    pool: &PgPool,
+    manga_id: Uuid,
+    number: i32,
+    title: Option<&str>,
+) -> AppResult<Chapter> {
+    let result = sqlx::query_as::<_, Chapter>(
+        r#"
+        INSERT INTO chapters (manga_id, number, title)
+        VALUES ($1, $2, $3)
+        RETURNING id, manga_id, number, title, page_count, created_at
+        "#,
+    )
+    .bind(manga_id)
+    .bind(number)
+    .bind(title)
+    .fetch_one(pool)
+    .await;
+
+    match result {
+        Ok(c) => Ok(c),
+        Err(e) if is_unique_violation(&e) => Err(crate::error::AppError::Conflict(format!(
+            "chapter {number} already exists for this manga"
+        ))),
+        Err(e) => Err(crate::error::AppError::Database(e)),
+    }
+}
+
+fn is_unique_violation(err: &sqlx::Error) -> bool {
+    if let sqlx::Error::Database(db_err) = err {
+        db_err.code().as_deref() == Some("23505")
+    } else {
+        false
+    }
+}