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/series_page.py

@@ -1,9 +1,12 @@
 """SeriesPage — ordered image membership for a series-kind Tag.
 
-A series IS a Tag with kind='series'; series_page gives it ordered pages.
-An image belongs to at most one series (UNIQUE image_id). Cover = the
-lowest page_number. page_number is an ordering key only (not unique) —
-reorder rewrites 1..N wholesale.
+A series IS a Tag with kind='series'; series_page gives it ordered pages,
+grouped into chapters (FC-6). An image belongs to at most one series
+(UNIQUE image_id) ⇒ at most one chapter. Reading order is
+(chapter.chapter_number, series_page.page_number): page_number orders pages
+WITHIN a chapter and is an ordering key only (not unique) — reorder rewrites
+1..N wholesale. stated_page carries the page number parsed from the source
+post (FC-6.2), nullable when unknown.
 """
 
 from datetime import datetime
@@ -21,12 +24,18 @@ class SeriesPage(Base):
     series_tag_id: Mapped[int] = mapped_column(
         ForeignKey("tag.id", ondelete="CASCADE"), nullable=False, index=True
     )
+    chapter_id: Mapped[int] = mapped_column(
+        ForeignKey("series_chapter.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
     image_id: Mapped[int] = mapped_column(
         ForeignKey("image_record.id", ondelete="CASCADE"),
         nullable=False,
         unique=True,
     )
     page_number: Mapped[int] = mapped_column(Integer, nullable=False)
+    stated_page: Mapped[int | None] = mapped_column(Integer, nullable=True)
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, server_default=func.now()
     )