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_chapter.py

@@ -1,28 +1,22 @@
-"""SeriesChapter — an ordered chapter/part within a series.
+"""SeriesChapter — a cosmetic chapter DIVIDER within a series (FC-6.x reframe).
 
-A series IS a Tag(kind='series'); a chapter groups ordered SeriesPages under it.
-Reading order is (chapter.chapter_number, series_page.page_number): chapter_number
-sets the order of chapters, page_number orders pages within a chapter.
+A series is ONE flat, series-global ordered run of SeriesPages. A chapter is NOT
+a container — it owns no pages. It is a labeled divider anchored to the page that
+BEGINS the chapter (anchor_page_id → series_page): "a new chapter starts here."
+A page's chapter is derived at read time as the nearest preceding divider.
 
-chapter_number is an ordering key only (not unique) — reorder rewrites 1..N
-wholesale, mirroring series_page.page_number, so a reorder can't transiently
-collide on a unique index.
+Dividers never affect page ordering or the series-global page numbers; they stay
+pinned to their anchor page across reorders. anchor_page_id is UNIQUE — at most
+one chapter begins at a given page — and FK-cascades, so removing the anchor page
+from the series drops the divider (the chapter merges into the preceding run).
 
-A chapter may be a placeholder (is_placeholder=True) — a reserved empty slot for
-a section the operator doesn't have yet; it holds no pages and shows as a gap in
-the reader. stated_page_start/end carry the page range parsed from the source
-post (FC-6.2), used to flag missing-page gaps; both are nullable when unknown.
-
-stated_part is the operator-facing "Part N" label (FC-6.4), separate from the
-positional chapter_number: chapter_number is auto-managed ordering (rewritten
-1..N on reorder/delete), while stated_part is the real installment number the
-operator types — e.g. a series authored from a post that is Part 2 of a story.
-Nullable when unset (the UI then falls back to showing chapter_number).
+title is the optional chapter name; stated_part is the optional operator-facing
+"Part N" label (shown instead of a derived ordinal when set).
 """
 
 from datetime import datetime
 
-from sqlalchemy import Boolean, DateTime, ForeignKey, Integer, Text, func
+from sqlalchemy import DateTime, ForeignKey, Integer, Text, func
 from sqlalchemy.orm import Mapped, mapped_column
 
 from .base import Base
@@ -35,14 +29,13 @@ class SeriesChapter(Base):
     series_tag_id: Mapped[int] = mapped_column(
         ForeignKey("tag.id", ondelete="CASCADE"), nullable=False, index=True
     )
-    chapter_number: Mapped[int] = mapped_column(Integer, nullable=False)
-    stated_part: Mapped[int | None] = mapped_column(Integer, nullable=True)
-    title: Mapped[str | None] = mapped_column(Text, nullable=True)
-    is_placeholder: Mapped[bool] = mapped_column(
-        Boolean, nullable=False, server_default="false"
+    anchor_page_id: Mapped[int] = mapped_column(
+        ForeignKey("series_page.id", ondelete="CASCADE"),
+        nullable=False,
+        unique=True,
     )
-    stated_page_start: Mapped[int | None] = mapped_column(Integer, nullable=True)
-    stated_page_end: Mapped[int | None] = mapped_column(Integer, nullable=True)
+    title: Mapped[str | None] = mapped_column(Text, nullable=True)
+    stated_part: Mapped[int | None] = mapped_column(Integer, nullable=True)
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, server_default=func.now()
     )