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>

frontend/src/lib/api/chapters.ts

@@ -0,0 +1,39 @@
+import { request, type Page } from './client';
+
+export type Chapter = {
+    id: string;
+    manga_id: string;
+    number: number;
+    title: string | null;
+    page_count: number;
+    created_at: string;
+};
+
+export type ChaptersPage = {
+    items: Chapter[];
+    page: Page;
+};
+
+export type ListOptions = {
+    limit?: number;
+    offset?: number;
+};
+
+export async function listChapters(
+    mangaId: string,
+    opts: ListOptions = {}
+): Promise<ChaptersPage> {
+    const params = new URLSearchParams();
+    if (opts.limit != null) params.set('limit', String(opts.limit));
+    if (opts.offset != null) params.set('offset', String(opts.offset));
+    const qs = params.toString();
+    return request<ChaptersPage>(
+        `/v1/mangas/${encodeURIComponent(mangaId)}/chapters${qs ? `?${qs}` : ''}`
+    );
+}
+
+export async function getChapter(mangaId: string, number: number): Promise<Chapter> {
+    return request<Chapter>(
+        `/v1/mangas/${encodeURIComponent(mangaId)}/chapters/${number}`
+    );
+}