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/services/series_match_service.py

@@ -18,7 +18,6 @@ from sqlalchemy.dialects.postgresql import insert as pg_insert
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from ..models import ImageRecord, Post, Tag, TagKind
-from ..models.series_chapter import SeriesChapter
 from ..models.series_page import SeriesPage
 from ..models.series_suggestion import SeriesSuggestion
 from ..models.tag import image_tag
@@ -144,9 +143,11 @@ class SeriesMatchService:
         return [t for t in rows if t]
 
     async def _series_max_stated_end(self, series_tag_id: int) -> int | None:
+        # Highest printed page across the series' pages (FC-6.x: stated pages
+        # live on series_page now; chapters are cosmetic dividers).
         return await self.session.scalar(
-            select(func.max(SeriesChapter.stated_page_end)).where(
-                SeriesChapter.series_tag_id == series_tag_id
+            select(func.max(SeriesPage.stated_page)).where(
+                SeriesPage.series_tag_id == series_tag_id
             )
         )
 
@@ -288,7 +289,7 @@ class SeriesMatchService:
             raise SeriesError(f"suggestion {suggestion_id} not found")
         if s.status != "pending":
             raise SeriesError(f"suggestion {suggestion_id} is already {s.status}")
-        out = await SeriesService(self.session).add_post_as_chapter(
+        out = await SeriesService(self.session).add_post(
             s.series_tag_id, s.post_id
         )
         s.status = "added"