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/src/repo/chapter.rs

@@ -12,15 +12,20 @@ pub async fn list_for_manga(
     limit: i64,
     offset: i64,
 ) -> AppResult<Vec<Chapter>> {
-    // Secondary sort by created_at gives duplicate-numbered chapters
-    // (multiple uploaders/translations of the same number) a stable
-    // order in lists and prev/next reader navigation.
+    // Display order = source-site order reversed. The crawler stamps
+    // `source_index` = position in the source DOM (0 = first = newest
+    // on this site, see migration 0021), so DESC puts the oldest
+    // chapter first and keeps the site's variant grouping and the
+    // placement of non-numeric entries (e.g. "notice. : Officials")
+    // intact. NULLS LAST keeps user-uploaded chapters (no source row)
+    // and rows that pre-date the migration below crawled rows; the
+    // (number, created_at) tail then orders them deterministically.
     let rows = sqlx::query_as::<_, Chapter>(
         r#"
         SELECT id, manga_id, number, title, page_count, created_at
         FROM chapters
         WHERE manga_id = $1
-        ORDER BY number ASC, created_at ASC
+        ORDER BY source_index DESC NULLS LAST, number ASC, created_at ASC
         LIMIT $2 OFFSET $3
         "#,
     )