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/migrations/0021_chapter_source_index.sql

@@ -0,0 +1,18 @@
+-- Capture each chapter's position in the source site's chapter list so
+-- the user-facing list can preserve site order: variants of the same
+-- chapter number (e.g. "Ch.14 : PH" next to "Ch.14 : Official") stay
+-- adjacent, and non-numeric entries like "notice. : Officials" land
+-- where the site placed them rather than clustering at the top under
+-- number = 0.
+--
+-- Lower source_index = closer to the top of the source DOM = newer
+-- chapter on this site (it renders newest-first). The list query
+-- reverses this with ORDER BY source_index DESC so the oldest chapter
+-- appears first in our UI.
+--
+-- NULL is the sentinel for user-uploaded chapters (no source row) and
+-- for crawled rows that pre-date this migration. The list query keeps
+-- the existing (number, created_at) tiebreak via NULLS LAST so those
+-- fall through to the prior behaviour until the next crawler tick
+-- populates the column.
+ALTER TABLE chapters ADD COLUMN source_index INTEGER;