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/crawler.rs

@@ -352,7 +352,14 @@ pub async fn sync_manga_chapters(
         .map(|c| c.source_chapter_key.clone())
         .collect();
 
-    for c in chapters {
+    for (idx, c) in chapters.iter().enumerate() {
+        // `source_index` captures the chapter's position in the source
+        // DOM (0 = first = newest on this site) so the list query can
+        // reverse it for the user-facing list — see migration 0021.
+        // Every sync overwrites the value on both branches, so a new
+        // chapter inserted at the top of the source shifts every other
+        // row down by one on the next tick.
+        let source_index = idx as i32;
         // Lookup is constrained by manga_id (via the chapters join) so a
         // source whose chapter slugs collide across mangas (e.g.
         // "chapter-1" appearing under two different mangas) attributes
@@ -382,14 +389,15 @@ pub async fn sync_manga_chapters(
                 // 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)
+                    INSERT INTO chapters (manga_id, number, title, page_count, source_index)
+                    VALUES ($1, $2, $3, 0, $4)
                     RETURNING id
                     "#,
                 )
                 .bind(manga_id)
                 .bind(c.number)
                 .bind(c.title.as_deref())
+                .bind(source_index)
                 .fetch_one(&mut *tx)
                 .await?;
                 sqlx::query(
@@ -408,8 +416,11 @@ pub async fn sync_manga_chapters(
                 diff.new += 1;
             }
             Some((chapter_id,)) => {
-                sqlx::query("UPDATE chapters SET title = $1 WHERE id = $2")
+                sqlx::query(
+                    "UPDATE chapters SET title = $1, source_index = $2 WHERE id = $3",
+                )
                     .bind(c.title.as_deref())
+                    .bind(source_index)
                     .bind(chapter_id)
                     .execute(&mut *tx)
                     .await?;