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>

frontend/src/routes/bookmarks/+page.svelte

@@ -20,11 +20,16 @@
     <ul class="bookmark-list" data-testid="bookmark-list">
         {#each bookmarks as b (b.id)}
             <li>
-                {#if b.chapter_id}
-                    <a href="/manga/{b.manga_id}/chapter/{b.chapter_id}">
-                        Chapter bookmark
+                {#if b.chapter_id && b.chapter_number != null}
+                    <a href="/manga/{b.manga_id}/chapter/{b.chapter_number}">
+                        Chapter {b.chapter_number}
                         {#if b.page}— page {b.page}{/if}
                     </a>
+                {:else if b.chapter_id}
+                    <!-- Chapter bookmark whose chapter was deleted; fall
+                         back to the manga overview rather than emit a
+                         broken link to a number we don't have. -->
+                    <a href="/manga/{b.manga_id}">Chapter bookmark (chapter removed)</a>
                 {:else}
                     <a href="/manga/{b.manga_id}">Manga bookmark</a>
                 {/if}