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>

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

@@ -135,11 +135,11 @@
         // navigation feels continuous in single mode. Harmless in
         // continuous mode (the reader just shows everything).
         const target = mode === 'single' ? `?page=last` : '';
-        void goto(`/manga/${manga.id}/chapter/${prevChapter.number}${target}`);
+        void goto(`/manga/${manga.id}/chapter/${prevChapter.id}${target}`);
     }
     function jumpToNextChapter() {
         if (!nextChapter) return;
-        void goto(`/manga/${manga.id}/chapter/${nextChapter.number}`);
+        void goto(`/manga/${manga.id}/chapter/${nextChapter.id}`);
     }
 
     function next() {

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

@@ -6,11 +6,10 @@ import type { PageLoad } from './$types';
 export const ssr = false;
 
 export const load: PageLoad = async ({ params, url }) => {
-    const number = Number(params.n);
     const [manga, chapter, pages, readProgress, chapterList] = await Promise.all([
         getManga(params.id),
-        getChapter(params.id, number),
-        getChapterPages(params.id, number),
+        getChapter(params.id, params.chapter_id),
+        getChapterPages(params.id, params.chapter_id),
         // `null` for guests or first-time openers — the reader uses
         // this to seed its session-local high-water mark.
         getMyReadProgressForManga(params.id),

frontend/src/routes/profile/history/+page.svelte

@@ -60,8 +60,8 @@
                 {#each progress as p (p.manga_id)}
                     <li class="entry">
                         <a
-                            href={p.chapter_number != null
-                                ? `/manga/${p.manga_id}/chapter/${p.chapter_number}`
+                            href={p.chapter_id != null
+                                ? `/manga/${p.manga_id}/chapter/${p.chapter_id}`
                                 : `/manga/${p.manga_id}`}
                             class="cover-link"
                             tabindex="-1"
@@ -89,9 +89,9 @@
                                 {p.manga_title}
                             </a>
                             <span class="target">
-                                {#if p.chapter_number != null}
+                                {#if p.chapter_id != null && p.chapter_number != null}
                                     <a
-                                        href="/manga/{p.manga_id}/chapter/{p.chapter_number}"
+                                        href="/manga/{p.manga_id}/chapter/{p.chapter_id}"
                                     >
                                         Continue Ch. {p.chapter_number}{#if p.page > 1} — page {p.page}{/if}
                                     </a>
@@ -185,7 +185,7 @@
                             <div class="meta">
                                 <a href="/manga/{u.manga_id}" class="title">{u.manga_title}</a>
                                 <span class="target">
-                                    <a href="/manga/{u.manga_id}/chapter/{u.chapter.number}">
+                                    <a href="/manga/{u.manga_id}/chapter/{u.chapter.id}">
                                         Chapter {u.chapter.number}{#if u.chapter.title}: {u.chapter.title}{/if}
                                     </a>
                                     <span class="muted">({u.chapter.page_count} pages)</span>