feat(series): chapter layer over series_page — backend (FC-6.1)

Adds an ordered chapter layer to series. Reading order becomes
(series_chapter.chapter_number, series_page.page_number); a chapter may be a
placeholder reserving a slot, and carries an optional parsed stated-page range
used to flag missing-page gaps. An image still lives in at most one series ⇒ one
chapter (image_id stays UNIQUE).

- models: series_chapter; series_page gains chapter_id (NOT NULL, cascade) +
  stated_page. Migration 0040 backfills every existing series into one
  auto-chapter holding its current flat pages — no data loss.
- SeriesService: chapter CRUD (create/update/reorder/delete/merge), page→chapter
  assignment, reorder_pages, chapter-aware set_cover; list_pages now returns
  chapters[] + gaps[] alongside a back-compat flat pages[]. Legacy series-wide
  reorder operates on the single default chapter and rejects multi-chapter series.
- API: chapter endpoints under /api/series/<tag>/chapters; POST pages accepts an
  optional chapter_id.
- TagService.merge now repoints series_chapter too, so a merged series' chapters
  (and their pages) survive the source tag's deletion instead of cascading away.
- Tests: new chapter suite; updated the 4 direct SeriesPage(...) constructions to
  supply chapter_id.

Frontend (chapter-aware manage view + reader) lands next; until then the
existing UI keeps working via the flat pages[] + single default chapter.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -18,6 +18,7 @@ from .patreon_failed_media import PatreonFailedMedia
 from .patreon_seen_media import PatreonSeenMedia
 from .post import Post
 from .post_attachment import PostAttachment
+from .series_chapter import SeriesChapter
 from .series_page import SeriesPage
 from .source import Source
 from .tag import Tag, TagKind, image_tag
@@ -39,6 +40,7 @@ __all__ = [
     "PatreonSeenMedia",
     "Post",
     "PostAttachment",
+    "SeriesChapter",
     "SeriesPage",
     "ImageRecord",
     "ImageProvenance",