feat(chapter): preserve source-site order in chapter list (0.52.0)

The user-facing chapter list ordered by (number ASC, created_at ASC),
which broke the source site's order in two ways: non-numeric entries
("notice. : Officials") parsed to number=0 and clustered at the top,
even though the site placed them mid-list, and variants sharing a
number ("Ch.14 : PH" / "Ch.14 : Official") were torn apart by the
created_at tiebreak.

Capture each chapter's position in the source DOM as `source_index`
(0 = first = newest on this site) on every crawler sync, including the
UPDATE branch so a new chapter prepended on the source shifts every
existing row down by one on the next tick. The list query reverses
this with `ORDER BY source_index DESC NULLS LAST, number ASC,
created_at ASC` so the oldest chapter appears first, variants stay
adjacent in the order the site shows them, and non-numeric entries
land where the site placed them. User-uploaded chapters and pre-
migration rows keep their NULL source_index and fall through to the
prior number/created_at tiebreak via NULLS LAST.

The reader's client-side `[...chapters].sort((a,b) => a.number - b.number)`
is dropped; prev/next now walks the server-ordered array positionally
so it traverses variants and non-numeric entries in display order.

Existing data populates on the next cron tick or via admin force-resync.

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

backend/tests/crawler_sync.rs

@@ -6,6 +6,7 @@
 
 use mangalord::crawler::source::{SourceChapterRef, SourceManga};
 use mangalord::repo::crawler::{self, ChapterDiff, UpsertStatus};
+use mangalord::repo::chapter as chapter_repo;
 use sqlx::PgPool;
 use uuid::Uuid;
 
@@ -961,3 +962,261 @@ async fn re_appearing_manga_clears_dropped_at(pool: PgPool) {
     assert!(dropped.0.is_none());
     assert_eq!(dropped.1, up.manga_id);
 }
+
+// ---- source_index: site-order preservation ----
+//
+// The user-facing chapter list reverses the source-site order so that
+// the oldest chapter appears first. The crawler records each row's DOM
+// position in `chapters.source_index` (0 = first in source DOM = newest
+// on this site) on every sync; the list query orders by source_index
+// DESC NULLS LAST, falling through to number/created_at for rows with
+// no source row (e.g. user uploads).
+
+#[sqlx::test(migrations = "./migrations")]
+async fn source_index_set_on_insert_matches_dom_order(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();
+
+    let chapters = vec![
+        SourceChapterRef {
+            source_chapter_key: "a".into(),
+            number: 30,
+            title: Some("Ch.30".into()),
+            url: "https://x.example/foo/a".into(),
+        },
+        SourceChapterRef {
+            source_chapter_key: "b".into(),
+            number: 29,
+            title: Some("Ch.29".into()),
+            url: "https://x.example/foo/b".into(),
+        },
+        SourceChapterRef {
+            source_chapter_key: "c".into(),
+            number: 28,
+            title: Some("Ch.28".into()),
+            url: "https://x.example/foo/c".into(),
+        },
+    ];
+    crawler::sync_manga_chapters(&pool, "target", up.manga_id, &chapters)
+        .await
+        .unwrap();
+
+    let rows: Vec<(String, Option<i32>)> = sqlx::query_as(
+        "SELECT cs.source_chapter_key, c.source_index \
+           FROM chapters c \
+           JOIN chapter_sources cs ON cs.chapter_id = c.id \
+          WHERE c.manga_id = $1 \
+          ORDER BY cs.source_chapter_key",
+    )
+    .bind(up.manga_id)
+    .fetch_all(&pool)
+    .await
+    .unwrap();
+    assert_eq!(
+        rows,
+        vec![
+            ("a".to_string(), Some(0)),
+            ("b".to_string(), Some(1)),
+            ("c".to_string(), Some(2)),
+        ],
+        "source_index reflects enumerate() position in the input slice",
+    );
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn source_index_rewritten_on_resync_when_new_chapter_prepended(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();
+
+    let first = vec![
+        SourceChapterRef {
+            source_chapter_key: "a".into(),
+            number: 1,
+            title: Some("Ch.1".into()),
+            url: "https://x.example/foo/a".into(),
+        },
+        SourceChapterRef {
+            source_chapter_key: "b".into(),
+            number: 2,
+            title: Some("Ch.2".into()),
+            url: "https://x.example/foo/b".into(),
+        },
+    ];
+    crawler::sync_manga_chapters(&pool, "target", up.manga_id, &first)
+        .await
+        .unwrap();
+
+    // Second sync: a brand-new chapter appears at the top of the source
+    // (newest first on the site). All existing rows must shift their
+    // source_index down by one so the display order stays correct.
+    let second = vec![
+        SourceChapterRef {
+            source_chapter_key: "new".into(),
+            number: 3,
+            title: Some("Ch.3".into()),
+            url: "https://x.example/foo/new".into(),
+        },
+        SourceChapterRef {
+            source_chapter_key: "a".into(),
+            number: 1,
+            title: Some("Ch.1".into()),
+            url: "https://x.example/foo/a".into(),
+        },
+        SourceChapterRef {
+            source_chapter_key: "b".into(),
+            number: 2,
+            title: Some("Ch.2".into()),
+            url: "https://x.example/foo/b".into(),
+        },
+    ];
+    crawler::sync_manga_chapters(&pool, "target", up.manga_id, &second)
+        .await
+        .unwrap();
+
+    let rows: Vec<(String, Option<i32>)> = sqlx::query_as(
+        "SELECT cs.source_chapter_key, c.source_index \
+           FROM chapters c \
+           JOIN chapter_sources cs ON cs.chapter_id = c.id \
+          WHERE c.manga_id = $1 \
+          ORDER BY cs.source_chapter_key",
+    )
+    .bind(up.manga_id)
+    .fetch_all(&pool)
+    .await
+    .unwrap();
+    assert_eq!(
+        rows,
+        vec![
+            ("a".to_string(), Some(1)),
+            ("b".to_string(), Some(2)),
+            ("new".to_string(), Some(0)),
+        ],
+        "new chapter takes index 0, existing rows shift down on UPDATE",
+    );
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn list_for_manga_returns_source_order_reversed(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();
+
+    // Site DOM order (top-down = newest-first):
+    //   ch11  (number = 11)
+    //   notice (number = 0, non-numeric label on the site)
+    //   ch10  (number = 10)
+    // Numbers deliberately disagree with DOM order: a number-based sort
+    // would put notice first, but the site places it between ch10 and
+    // ch11. Reversed-DOM display should yield [ch10, notice, ch11].
+    let chapters = vec![
+        SourceChapterRef {
+            source_chapter_key: "ch11".into(),
+            number: 11,
+            title: Some("Ch.11 : Official".into()),
+            url: "https://x.example/foo/11".into(),
+        },
+        SourceChapterRef {
+            source_chapter_key: "notice".into(),
+            number: 0,
+            title: Some("notice. : Officials".into()),
+            url: "https://x.example/foo/notice".into(),
+        },
+        SourceChapterRef {
+            source_chapter_key: "ch10".into(),
+            number: 10,
+            title: Some("Ch.10 : Official".into()),
+            url: "https://x.example/foo/10".into(),
+        },
+    ];
+    crawler::sync_manga_chapters(&pool, "target", up.manga_id, &chapters)
+        .await
+        .unwrap();
+
+    let listed = chapter_repo::list_for_manga(&pool, up.manga_id, 50, 0)
+        .await
+        .unwrap();
+    let keys: Vec<String> = listed
+        .iter()
+        .map(|c| c.title.clone().unwrap_or_default())
+        .collect();
+    assert_eq!(
+        keys,
+        vec![
+            "Ch.10 : Official".to_string(),
+            "notice. : Officials".to_string(),
+            "Ch.11 : Official".to_string(),
+        ],
+        "list returns chapters in reversed source-DOM order, so the \
+         oldest appears first and non-numeric entries land where the \
+         site placed them",
+    );
+}
+
+#[sqlx::test(migrations = "./migrations")]
+async fn list_for_manga_places_null_source_index_last(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();
+
+    // Crawled chapters get source_index 0 and 1; the upload path leaves
+    // it NULL. NULLS LAST plus the (number, created_at) tail means the
+    // upload sits after both crawled rows even though its number is in
+    // the middle.
+    let crawled = vec![
+        SourceChapterRef {
+            source_chapter_key: "a".into(),
+            number: 1,
+            title: Some("Ch.1".into()),
+            url: "https://x.example/foo/a".into(),
+        },
+        SourceChapterRef {
+            source_chapter_key: "b".into(),
+            number: 3,
+            title: Some("Ch.3".into()),
+            url: "https://x.example/foo/b".into(),
+        },
+    ];
+    crawler::sync_manga_chapters(&pool, "target", up.manga_id, &crawled)
+        .await
+        .unwrap();
+
+    chapter_repo::create(&pool, up.manga_id, 2, Some("User upload Ch.2"), None)
+        .await
+        .unwrap();
+
+    let listed = chapter_repo::list_for_manga(&pool, up.manga_id, 50, 0)
+        .await
+        .unwrap();
+    let titles: Vec<String> = listed
+        .iter()
+        .map(|c| c.title.clone().unwrap_or_default())
+        .collect();
+    assert_eq!(
+        titles,
+        vec![
+            "Ch.3".to_string(),
+            "Ch.1".to_string(),
+            "User upload Ch.2".to_string(),
+        ],
+        "crawled rows ordered by reversed source_index; user upload \
+         (NULL source_index) falls through to the end",
+    );
+}