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/tests/crawler_sync.rs

@@ -232,6 +232,82 @@ async fn sync_chapters_adds_new_refreshes_existing_and_drops_vanished(pool: PgPo
     assert!(dropped.0.is_some(), "ch2 should be soft-dropped");
 }
 
+/// Real-world sources publish multiple chapters at the same number
+/// (different uploaders, translator notes, re-releases). After the
+/// (manga_id, number) UNIQUE drop in 0013, each `SourceChapterRef`
+/// becomes its own `chapters` row even when the parsed number matches
+/// — chapter identity is now the chapter id, not the number.
+#[sqlx::test(migrations = "./migrations")]
+async fn sync_chapters_keeps_duplicate_numbered_chapters_as_separate_rows(pool: PgPool) {
+    crawler::ensure_source(&pool, "target", "T", "https://x.example")
+        .await
+        .unwrap();
+    let m = sample_manga("foo", "Foo Manga", "hash-1");
+    let up = crawler::upsert_manga_from_source(&pool, "target", "https://x.example/foo", &m)
+        .await
+        .unwrap();
+
+    // Two distinct uploads of Ch.52 (different uploaders → different
+    // URLs/keys, same parsed number) plus a notice/hiatus row that
+    // parses to number=0 alongside a real chapter at number 1.
+    let chapters = vec![
+        SourceChapterRef {
+            source_chapter_key: "br_chapter-A".into(),
+            number: 52,
+            title: Some("Ch.52 : Official".into()),
+            url: "https://x.example/foo/A/pg-1/".into(),
+        },
+        SourceChapterRef {
+            source_chapter_key: "br_chapter-B".into(),
+            number: 52,
+            title: Some("Ch.52 : Official (alt)".into()),
+            url: "https://x.example/foo/B/pg-1/".into(),
+        },
+        SourceChapterRef {
+            source_chapter_key: "br_chapter-NOTICE".into(),
+            number: 0,
+            title: Some("hitaus.".into()),
+            url: "https://x.example/foo/notice/pg-1/".into(),
+        },
+        SourceChapterRef {
+            source_chapter_key: "br_chapter-1".into(),
+            number: 1,
+            title: Some("Ch.1 : Official".into()),
+            url: "https://x.example/foo/1/pg-1/".into(),
+        },
+    ];
+
+    let diff = crawler::sync_manga_chapters(&pool, "target", up.manga_id, &chapters)
+        .await
+        .unwrap();
+    assert_eq!(
+        diff,
+        ChapterDiff {
+            new: 4,
+            refreshed: 0,
+            dropped: 0
+        },
+        "every source ref yields a new chapter row"
+    );
+
+    let rows: (i64,) =
+        sqlx::query_as("SELECT COUNT(*) FROM chapters WHERE manga_id = $1")
+            .bind(up.manga_id)
+            .fetch_one(&pool)
+            .await
+            .unwrap();
+    assert_eq!(rows.0, 4, "4 distinct chapter rows even with duplicate numbers");
+
+    let ch52_count: (i64,) = sqlx::query_as(
+        "SELECT COUNT(*) FROM chapters WHERE manga_id = $1 AND number = 52",
+    )
+    .bind(up.manga_id)
+    .fetch_one(&pool)
+    .await
+    .unwrap();
+    assert_eq!(ch52_count.0, 2, "both Ch.52 uploads survive as separate rows");
+}
+
 #[sqlx::test(migrations = "./migrations")]
 async fn mark_dropped_mangas_only_drops_unseen(pool: PgPool) {
     crawler::ensure_source(&pool, "target", "T", "https://x.example")