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>

tests/test_series_match_service.py

@@ -79,7 +79,7 @@ async def test_match_is_idempotent_under_unique(db):
 
 
 @pytest.mark.asyncio
-async def test_accept_adds_chapter_and_marks_added(db):
+async def test_accept_appends_pages_and_marks_added(db):
     artist = await _artist(db, "Accept Artist")
     a = await _post_with_images(db, artist, "Saga pages 1-4", "a", 3)
     sid = await _series_from(db, a)
@@ -90,7 +90,8 @@ async def test_accept_adds_chapter_and_marks_added(db):
 
     await svc.accept(sug["id"])
     data = await SeriesService(db).list_pages(sid)
-    assert len(data["chapters"]) == 2          # b became a 2nd chapter
+    # b's pages were appended to the series' flat run (3 from a + 3 from b).
+    assert len(data["pages"]) == 6
     status = await db.scalar(
         select(SeriesSuggestion.status).where(SeriesSuggestion.id == sug["id"])
     )