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_series_model.py

@@ -3,6 +3,7 @@ from sqlalchemy import select
 from sqlalchemy.exc import IntegrityError
 
 from backend.app.models import ImageRecord, TagKind
+from backend.app.models.series_chapter import SeriesChapter
 from backend.app.models.series_page import SeriesPage
 from backend.app.services.tag_service import TagService
 
@@ -24,10 +25,20 @@ async def _img(db):
 
 
 @pytest.mark.asyncio
+async def _chapter(db, series_tag_id):
+    ch = SeriesChapter(series_tag_id=series_tag_id, chapter_number=1)
+    db.add(ch)
+    await db.flush()
+    return ch.id
+
+
 async def test_series_page_roundtrip_and_unique_image(db):
     s = await TagService(db).find_or_create("Vol1", TagKind.series)
     i1 = await _img(db)
-    db.add(SeriesPage(series_tag_id=s.id, image_id=i1, page_number=1))
+    ch = await _chapter(db, s.id)
+    db.add(SeriesPage(
+        series_tag_id=s.id, chapter_id=ch, image_id=i1, page_number=1
+    ))
     await db.flush()
     got = await db.scalar(
         select(SeriesPage.page_number).where(SeriesPage.image_id == i1)
@@ -35,6 +46,9 @@ async def test_series_page_roundtrip_and_unique_image(db):
     assert got == 1
     # UNIQUE image_id: same image can't be a page twice (any series).
     s2 = await TagService(db).find_or_create("Vol2", TagKind.series)
-    db.add(SeriesPage(series_tag_id=s2.id, image_id=i1, page_number=1))
+    ch2 = await _chapter(db, s2.id)
+    db.add(SeriesPage(
+        series_tag_id=s2.id, chapter_id=ch2, image_id=i1, page_number=1
+    ))
     with pytest.raises(IntegrityError):
         await db.flush()