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>

tests/test_cleanup_service.py

@@ -9,6 +9,7 @@ import pytest
 from sqlalchemy import func, select
 
 from backend.app.models import Artist, ImageRecord, Tag, TagKind
+from backend.app.models.series_chapter import SeriesChapter
 from backend.app.models.series_page import SeriesPage
 from backend.app.models.tag import image_tag
 from backend.app.services import cleanup_service
@@ -376,7 +377,12 @@ def test_reset_content_tagging_deletes_content_keeps_fandom_series(db_sync, tmp_
         {"image_record_id": img.id, "tag_id": c.id},
         {"image_record_id": img.id, "tag_id": s.id},  # series membership
     ]))
-    db_sync.add(SeriesPage(series_tag_id=s.id, image_id=img.id, page_number=1))
+    ch = SeriesChapter(series_tag_id=s.id, chapter_number=1)
+    db_sync.add(ch)
+    db_sync.flush()
+    db_sync.add(SeriesPage(
+        series_tag_id=s.id, chapter_id=ch.id, image_id=img.id, page_number=1
+    ))
     db_sync.commit()
 
     result = cleanup_service.reset_content_tagging(db_sync, dry_run=False)