feat(series): flat series sequence + cosmetic chapter dividers (#789 Phase 1)

Reframe a series from "ordered chapters that own pages" to ONE flat,
series-global ordered run of pages with optional cosmetic chapter DIVIDERS
over it. A chapter no longer wraps content — it's a labeled divider anchored
to the page that begins it; a page's chapter is derived as the nearest
preceding divider. This is what lets installments assembled from multiple
sources sit in one continuous, correctly-numbered sequence (operator's
Goblin Juice case).

- migration 0047: flatten each series to a series-global page_number
  (preserving today's reading order); convert each existing chapter to a
  divider anchored at its first page (keeping title/stated_part); drop
  series_page.chapter_id; reshape series_chapter (anchor_page_id UNIQUE FK,
  drop chapter_number/is_placeholder/stated_page_start/end). Loss-safe for
  content; drops empty placeholder chapters + a redundant page-1 divider.
- series_page: page_number is now the series-global order; no chapter_id.
- series_chapter: anchored divider (anchor_page_id, title, stated_part).
- series_service: flat list_pages (one run + derived dividers + per-page
  source_post + part_gaps), series-wide reorder/renumber, divider CRUD
  (create/update/move/delete); retired per-chapter reorder/merge/placement.
- api/tags: drop chapter_id from add; /chapters endpoints are divider
  create/update/delete (removed chapter reorder/merge/page-reorder).
- series_match_service: series "end" reads max(series_page.stated_page);
  accept appends via add_post. tag_service series-merge appends src's pages
  after tgt's max so the merged series stays one clean run.
- frontend: seriesManage store + SeriesManageView → one continuous
  drag-reorder grid with inline divider bars + series-global page numbers;
  reader walks the flat run, headings from dividers; PostSeriesMenu copy.
- tests reworked across the series suite for the divider model.

Phase 2 (pending staging for add-from-post) is separate.

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

backend/app/models/series_page.py

@@ -1,12 +1,15 @@
 """SeriesPage — ordered image membership for a series-kind Tag.
 
-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.
+A series IS a Tag with kind='series'; series_page gives it a SINGLE flat,
+series-global ordered run of pages (FC-6.x divider reframe). An image belongs to
+at most one series (UNIQUE image_id). Reading order is `page_number` alone — a
+series-wide ordering key (not unique), rewritten 1..N wholesale on reorder so a
+reorder can't transiently collide on an index.
+
+Chapters are cosmetic DIVIDERS anchored to a page (see SeriesChapter); they do
+NOT own pages, so there is no chapter_id here — a page's chapter is derived at
+read time as the nearest preceding divider. stated_page carries the printed page
+number parsed from the source post, nullable when unknown.
 """
 
 from datetime import datetime
@@ -24,11 +27,6 @@ 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,