feat: route reader by chapter id, allow duplicate-numbered chapters (0.24.0)

Real-world sources publish multiple chapters at the same number:
different scanlators ("Ch.52 from bloomingdale" + "Ch.52 from mina"),
translator notices and farewells, alt-translations. The (manga_id,
number) UNIQUE constraint from 0001 silently collapsed all of those
into a single row via the upsert path in repo::crawler. Migration 0013
drops the constraint; sync_manga_chapters now plain-INSERTs each
SourceChapterRef so every parsed chapter survives as its own row.

Identity moves from the (manga_id, number) tuple to the chapter UUID:

- `GET /api/v1/mangas/:manga_id/chapters/:chapter_id` (replaces :number)
- `GET /api/v1/mangas/:manga_id/chapters/:chapter_id/pages`
- `repo::chapter::find_by_id_in_manga` (replaces find_by_manga_and_number)
- Frontend reader route renamed to `/manga/[id]/chapter/[chapter_id]`
- Chapter links throughout (manga page list, continue-reading CTA,
  reader prev/next, history rows, bookmark cards) use chapter.id
- API clients getChapter/getChapterPages take a chapter id string

read_progress + bookmarks already FK chapter_id; they only enrich with
chapter_number for display, which is preserved.

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

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

@@ -29,6 +29,9 @@
             ? chapters.find((c) => c.id === readProgress.chapter_id) ?? null
             : null
     );
+    /** Reader link target — always the chapter id when we have one,
+     *  even for chapters past the loaded `chapters` list page. */
+    const continueChapterId = $derived(readProgress?.chapter_id ?? null);
     const continueChapterNumber = $derived(
         continueChapter?.number ?? readProgress?.chapter_number ?? null
     );
@@ -351,10 +354,10 @@
 
     <section aria-label="chapters">
         <h2>Chapters</h2>
-        {#if continueChapterNumber != null}
+        {#if continueChapterId != null && continueChapterNumber != null}
             <a
                 class="continue"
-                href="/manga/{manga.id}/chapter/{continueChapterNumber}"
+                href="/manga/{manga.id}/chapter/{continueChapterId}"
                 data-testid="continue-reading"
             >
                 <span class="continue-label">Continue reading</span>
@@ -372,7 +375,7 @@
             <ol class="chapter-list" data-testid="chapter-list">
                 {#each chapters as c (c.id)}
                     <li>
-                        <a href="/manga/{manga.id}/chapter/{c.number}">
+                        <a href="/manga/{manga.id}/chapter/{c.id}">
                             Chapter {c.number}{#if c.title}: {c.title}{/if}
                         </a>
                         <span class="pages">({c.page_count} pages)</span>