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_model.py

@@ -25,20 +25,10 @@ async def _img(db):
 
 
 @pytest.mark.asyncio
-async def _chapter(db, series_tag_id):
-    ch = SeriesChapter(series_tag_id=series_tag_id, chapter_number=1)
-    db.add(ch)
-    await db.flush()
-    return ch.id
-
-
 async def test_series_page_roundtrip_and_unique_image(db):
     s = await TagService(db).find_or_create("Vol1", TagKind.series)
     i1 = await _img(db)
-    ch = await _chapter(db, s.id)
-    db.add(SeriesPage(
-        series_tag_id=s.id, chapter_id=ch, image_id=i1, page_number=1
-    ))
+    db.add(SeriesPage(series_tag_id=s.id, image_id=i1, page_number=1))
     await db.flush()
     got = await db.scalar(
         select(SeriesPage.page_number).where(SeriesPage.image_id == i1)
@@ -46,9 +36,21 @@ async def test_series_page_roundtrip_and_unique_image(db):
     assert got == 1
     # UNIQUE image_id: same image can't be a page twice (any series).
     s2 = await TagService(db).find_or_create("Vol2", TagKind.series)
-    ch2 = await _chapter(db, s2.id)
-    db.add(SeriesPage(
-        series_tag_id=s2.id, chapter_id=ch2, image_id=i1, page_number=1
-    ))
+    db.add(SeriesPage(series_tag_id=s2.id, image_id=i1, page_number=1))
+    with pytest.raises(IntegrityError):
+        await db.flush()
+
+
+@pytest.mark.asyncio
+async def test_chapter_divider_anchor_is_unique(db):
+    # A chapter divider anchors to the page that begins it; one divider per page.
+    s = await TagService(db).find_or_create("Vol3", TagKind.series)
+    i1 = await _img(db)
+    p = SeriesPage(series_tag_id=s.id, image_id=i1, page_number=1)
+    db.add(p)
+    await db.flush()
+    db.add(SeriesChapter(series_tag_id=s.id, anchor_page_id=p.id))
+    await db.flush()
+    db.add(SeriesChapter(series_tag_id=s.id, anchor_page_id=p.id))
     with pytest.raises(IntegrityError):
         await db.flush()