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/mod.rs

@@ -1,4 +1,5 @@
 pub mod api_token;
+pub mod chapter;
 pub mod manga;
 pub mod session;
 pub mod user;