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>

backend/src/repo/crawler.rs

@@ -332,15 +332,15 @@ pub async fn sync_manga_chapters(
 
         match existing {
             None => {
-                // New chapter row. The (manga_id, number) unique
-                // constraint protects against re-inserts if the same
-                // number arrives via a different source_chapter_key.
+                // New chapter row. As of 0013 there's no (manga_id,
+                // number) UNIQUE, so duplicate-numbered chapters from
+                // the source (different uploaders, notices, alt
+                // translations) each get their own row — chapter
+                // identity is the UUID, not the number.
                 let (chapter_id,): (Uuid,) = sqlx::query_as(
                     r#"
                     INSERT INTO chapters (manga_id, number, title, page_count)
                     VALUES ($1, $2, $3, 0)
-                    ON CONFLICT (manga_id, number) DO UPDATE
-                    SET title = EXCLUDED.title
                     RETURNING id
                     "#,
                 )