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/repo/bookmark.rs

@@ -3,7 +3,7 @@
 use sqlx::PgPool;
 use uuid::Uuid;
 
-use crate::domain::Bookmark;
+use crate::domain::{Bookmark, BookmarkSummary};
 use crate::error::{AppError, AppResult};
 
 pub async fn create(
@@ -36,18 +36,31 @@ pub async fn create(
     }
 }
 
+/// Returns the user's bookmarks enriched with each chapter's number
+/// (LEFT JOINed; `chapter_number` is `null` for manga-level bookmarks
+/// or for chapter bookmarks whose chapter has since been deleted —
+/// `bookmarks.chapter_id` is `ON DELETE SET NULL`). The frontend uses
+/// the number to build reader URLs, which are keyed on number, not id.
 pub async fn list_for_user(
     pool: &PgPool,
     user_id: Uuid,
     limit: i64,
     offset: i64,
-) -> AppResult<Vec<Bookmark>> {
-    let rows = sqlx::query_as::<_, Bookmark>(
+) -> AppResult<Vec<BookmarkSummary>> {
+    let rows = sqlx::query_as::<_, BookmarkSummary>(
         r#"
-        SELECT id, user_id, manga_id, chapter_id, page, created_at
-        FROM bookmarks
-        WHERE user_id = $1
-        ORDER BY created_at DESC
+        SELECT
+            b.id,
+            b.user_id,
+            b.manga_id,
+            b.chapter_id,
+            c.number AS chapter_number,
+            b.page,
+            b.created_at
+        FROM bookmarks b
+        LEFT JOIN chapters c ON c.id = b.chapter_id
+        WHERE b.user_id = $1
+        ORDER BY b.created_at DESC
         LIMIT $2 OFFSET $3
         "#,
     )