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/domain/bookmark.rs

@@ -12,3 +12,19 @@ pub struct Bookmark {
     pub page: Option<i32>,
     pub created_at: DateTime<Utc>,
 }
+
+/// `Bookmark` enriched with the chapter's reader-facing number, so the
+/// frontend can build `/manga/{id}/chapter/{number}` links without an
+/// extra round-trip per bookmark. Populated by a LEFT JOIN; `null` for
+/// manga-level bookmarks or for chapter bookmarks whose chapter has
+/// since been deleted (`chapter_id` is `ON DELETE SET NULL`).
+#[derive(Debug, Clone, Serialize, FromRow)]
+pub struct BookmarkSummary {
+    pub id: Uuid,
+    pub user_id: Uuid,
+    pub manga_id: Uuid,
+    pub chapter_id: Option<Uuid>,
+    pub chapter_number: Option<i32>,
+    pub page: Option<i32>,
+    pub created_at: DateTime<Utc>,
+}