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/src/api/bookmarks.rs

@@ -12,7 +12,7 @@ use uuid::Uuid;
 use crate::api::pagination::PagedResponse;
 use crate::app::AppState;
 use crate::auth::extractor::CurrentUser;
-use crate::domain::Bookmark;
+use crate::domain::{Bookmark, BookmarkSummary};
 use crate::error::{AppError, AppResult};
 use crate::repo;
 
@@ -95,7 +95,7 @@ async fn list_me(
     State(state): State<AppState>,
     CurrentUser(user): CurrentUser,
     Query(params): Query<ListParams>,
-) -> AppResult<Json<PagedResponse<Bookmark>>> {
+) -> AppResult<Json<PagedResponse<BookmarkSummary>>> {
     let limit = params.limit.clamp(1, 200);
     let offset = params.offset.max(0);
     let items = repo::bookmark::list_for_user(&state.db, user.id, limit, offset).await?;