bugfix: bookmark chapter links use chapter number, not UUID

The reader route is keyed on chapter number (URL `/manga/{id}/chapter/{n}`,
loaded via `Number(params.n)`), but the bookmarks list was building
hrefs from `chapter_id` (a UUID). Following any chapter bookmark
produced a NaN load on the reader page.

Fix at the API layer so every consumer of /me/bookmarks gets the
information without a follow-up round-trip per bookmark.

- domain::BookmarkSummary: new type, `Bookmark` plus
  `chapter_number: Option<i32>`. Populated by a LEFT JOIN on chapters
  so manga-level bookmarks come back with `chapter_number = null` and
  chapter-level ones get the value. `Bookmark` itself stays minimal
  for POST / DELETE responses.
- repo::bookmark::list_for_user returns Vec<BookmarkSummary>.
- api::bookmarks::list_me returns PagedResponse<BookmarkSummary>.
- Frontend `Bookmark` type carries an optional `chapter_number`.
- /bookmarks page builds `/manga/{manga_id}/chapter/{chapter_number}`
  for chapter bookmarks, falling back to the manga overview if the
  chapter has been deleted out from under the bookmark (chapter_id is
  ON DELETE SET NULL, so this is a real edge case).

New test asserts both branches of the JOIN: a chapter-level bookmark
comes back with the right chapter_number and page, a manga-level one
has a null chapter_number.

Lockstep version bump to 0.9.2.

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

backend/Cargo.lock

@@ -1033,7 +1033,7 @@ checksum = "5e5032e24019045c762d3c0f28f5b6b8bbf38563a65908389bf7978758920897"
 
 [[package]]
 name = "mangalord"
-version = "0.9.0"
+version = "0.9.1"
 dependencies = [
  "anyhow",
  "argon2",