feat: bookmarks (CRUD + per-user listing + frontend toggle)

Backend:
- Migration 0004_bookmarks_unique.sql adds a partial unique index on
  (user_id, manga_id) WHERE chapter_id IS NULL. The 0001 UNIQUE
  constraint over (user_id, manga_id, chapter_id) doesn't block dupes
  when chapter_id is NULL under Postgres's default NULLS DISTINCT, so a
  user could otherwise bookmark the same manga twice at the manga
  level. Chapter-level dupes are still caught by the 0001 constraint.
- repo::bookmark with create / list_for_user / find_owner / delete.
  create catches the 23505 unique violation and surfaces it as
  AppError::Conflict so handlers return a clean 409.
- POST /api/v1/bookmarks { manga_id, chapter_id?, page? } — CurrentUser
  required. Pre-validates the manga exists (404 if not) and, when
  chapter_id is supplied, that the chapter belongs to that manga (also
  404), so FK violations can't bubble up as 500s.
- DELETE /api/v1/bookmarks/{id} — owner-only. 404 if unknown, 403 if it
  exists for another user, 204 on success. Idempotent: deleting an
  already-deleted bookmark is 404, not 500.
- GET /api/v1/me/bookmarks — paged envelope, sorted by created_at DESC,
  scoped to the current user so the URL itself can't be used to peek at
  someone else's bookmarks.

Integration coverage in tests/api_bookmarks.rs (9 cases): create+list
returns only own; duplicate manga-level bookmark → 409; unknown manga
→ 404; unauthenticated POST → 401; user A cannot delete user B's
bookmark (403); unknown delete → 404; double-delete → 404, not 500;
/me/bookmarks requires auth; paged envelope shape on empty list.

Frontend:
- lib/api/bookmarks.ts with createBookmark / deleteBookmark /
  listMyBookmarks. listMyBookmarksOrEmpty wraps the 401 case so pages
  can render anonymously without try/catch boilerplate.
- /manga/[id] overview: pre-loads the user's bookmark list in its load
  function and renders either:
  - "★ Bookmarked" / "☆ Bookmark" toggle with aria-pressed when authed;
    click POSTs or DELETEs and mutates a local working copy of the
    bookmark list (optimistic UI without re-fetching);
  - or a "Sign in to bookmark" link for anonymous users.
- /bookmarks page lists the current user's bookmarks (chapter-level
  bookmarks link into the reader, manga-level back to the overview).
  Anonymous users see a sign-in prompt instead of a 401 page.

E2E in e2e/bookmarks.spec.ts (3 cases): authed toggle round-trip
(bookmark, see in /bookmarks list, unbookmark); anonymous user gets the
sign-in CTA on the overview; anonymous /bookmarks shows the sign-in
prompt. Existing reader.spec.ts updated for the new
bookmark-signin/toggle test IDs.

Lockstep version bump to 0.7.0.

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

frontend/src/routes/manga/[id]/+page.ts

@@ -1,13 +1,15 @@
 import { getManga } from '$lib/api/mangas';
 import { listChapters } from '$lib/api/chapters';
+import { listMyBookmarksOrEmpty } from '$lib/api/bookmarks';
 import type { PageLoad } from './$types';
 
 export const ssr = false;
 
 export const load: PageLoad = async ({ params }) => {
-    const [manga, chapters] = await Promise.all([
+    const [manga, chapters, bookmarks] = await Promise.all([
         getManga(params.id),
-        listChapters(params.id)
+        listChapters(params.id),
+        listMyBookmarksOrEmpty()
     ]);
-    return { manga, chapters: chapters.items };
+    return { manga, chapters: chapters.items, bookmarks: bookmarks.items };
 };