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_from_post.py

@@ -1,9 +1,11 @@
-"""FC-6.2 — promote a post to a series + add a post as a chapter + browse list."""
+"""FC-6.x — promote a post to a series + add a post's pages + browse list."""
 
 import pytest
+from sqlalchemy import select
 
 from backend.app.models import Artist, ImageRecord, Post, TagKind
-from backend.app.services.series_service import SeriesService
+from backend.app.models.series_page import SeriesPage
+from backend.app.services.series_service import SeriesError, SeriesService
 from backend.app.services.tag_service import TagService
 
 pytestmark = pytest.mark.integration
@@ -53,12 +55,10 @@ async def test_promote_post_to_series(db):
     assert out["name"] == "My Comic pages 1-3"
 
     data = await svc.list_pages(out["series_tag_id"])
-    assert len(data["chapters"]) == 1
-    ch = data["chapters"][0]
-    assert ch["stated_page_start"] == 1
-    assert ch["stated_page_end"] == 3
-    assert [p["image_id"] for p in ch["pages"]] == imgs  # capture order
-    assert [p["stated_page"] for p in ch["pages"]] == [1, 2, 3]
+    # one flat run in capture order, no dividers, stated pages parsed 1..3
+    assert [p["image_id"] for p in data["pages"]] == imgs
+    assert [p["stated_page"] for p in data["pages"]] == [1, 2, 3]
+    assert data["dividers"] == []
 
 
 @pytest.mark.asyncio
@@ -66,31 +66,35 @@ async def test_promote_requires_images(db):
     svc = SeriesService(db)
     artist = await _artist(db, "Empty Artist")
     post = await _post(db, artist, "No images", "pp2")
-    from backend.app.services.series_service import SeriesError
     with pytest.raises(SeriesError):
         await svc.promote_post_to_series(post.id)
 
 
 @pytest.mark.asyncio
-async def test_add_post_as_chapter_slots_by_stated_page(db):
+async def test_add_post_appends_pages_to_the_run(db):
     svc = SeriesService(db)
     artist = await _artist(db, "Story Artist")
     sid = (await TagService(db).find_or_create("Story", TagKind.series)).id
-    # An existing chapter stating pages 9-12.
-    await svc.create_chapter(sid, stated_page_start=9, stated_page_end=12)
-    # A post stating pages 1-4 should slot BEFORE the 9-12 chapter.
-    post = await _post(db, artist, "Story pages 1-4", "pp3")
-    await _post_images(db, post, artist, 4)
-    out = await svc.add_post_as_chapter(sid, post.id)
-    assert out["added"] == 4
+    # Seed the series with two existing pages.
+    seed_post = await _post(db, artist, "seed", "pp3a")
+    seed = await _post_images(db, seed_post, artist, 2)
+    await svc.add_images(sid, seed)
+    # A later post's pages append to the END (operator orders afterward).
+    post = await _post(db, artist, "Story pages 9-11", "pp3b")
+    imgs = await _post_images(db, post, artist, 3)
+    out = await svc.add_post(sid, post.id)
+    assert out["added"] == 3
 
-    data = await svc.list_pages(sid)
-    chapters = data["chapters"]
-    assert chapters[0]["stated_page_start"] == 1   # new one is first
-    assert chapters[1]["stated_page_start"] == 9
-    # gap 5-8 flagged between them
-    assert data["gaps"][0]["start"] == 5
-    assert data["gaps"][0]["end"] == 8
+    rows = (
+        await db.execute(
+            select(SeriesPage.image_id, SeriesPage.stated_page)
+            .where(SeriesPage.series_tag_id == sid)
+            .order_by(SeriesPage.page_number)
+        )
+    ).all()
+    assert [r.image_id for r in rows] == seed + imgs
+    stated = {r.image_id: r.stated_page for r in rows}
+    assert [stated[i] for i in imgs] == [9, 10, 11]
 
 
 @pytest.mark.asyncio
@@ -104,61 +108,32 @@ async def test_list_series_cards(db):
     rows = await svc.list_series()
     card = next(r for r in rows if r["id"] == out["series_tag_id"])
     assert card["name"] == "Card Comic pages 1-2"
-    assert card["chapter_count"] == 1
+    assert card["chapter_count"] == 0  # a flat promote has no dividers
     assert card["page_count"] == 2
     assert card["artist_name"] == "Card Artist"
     assert card["cover_thumbnail_url"]
     assert card["has_gap"] is False
 
     # artist filter keeps it; a different artist id drops it.
-    assert any(r["id"] == out["series_tag_id"] for r in await svc.list_series(artist_id=artist.id))
-    assert all(r["id"] != out["series_tag_id"] for r in await svc.list_series(artist_id=artist.id + 99999))
-
-
-# --- FC-6.4: stated_part, part_gaps, source_post label ---------------------
+    assert any(
+        r["id"] == out["series_tag_id"]
+        for r in await svc.list_series(artist_id=artist.id)
+    )
+    assert all(
+        r["id"] != out["series_tag_id"]
+        for r in await svc.list_series(artist_id=artist.id + 99999)
+    )
 
 
 @pytest.mark.asyncio
-async def test_update_chapter_sets_and_clears_stated_part(db):
-    svc = SeriesService(db)
-    sid = (await TagService(db).find_or_create("Part Series", TagKind.series)).id
-    ch = await svc.create_chapter(sid)
-    # Set the installment to Part 2 (chapter_number stays its positional value).
-    await svc.update_chapter(sid, ch["id"], stated_part=2, set_part=True)
-    data = await svc.list_pages(sid)
-    assert data["chapters"][0]["stated_part"] == 2
-    assert data["chapters"][0]["chapter_number"] == 1
-    # Clearing it writes NULL back.
-    await svc.update_chapter(sid, ch["id"], stated_part=None, set_part=True)
-    data = await svc.list_pages(sid)
-    assert data["chapters"][0]["stated_part"] is None
-
-
-@pytest.mark.asyncio
-async def test_part_gaps_flagged_from_stated_part(db):
-    svc = SeriesService(db)
-    sid = (await TagService(db).find_or_create("Gappy Series", TagKind.series)).id
-    c1 = await svc.create_chapter(sid)
-    c3 = await svc.create_chapter(sid)
-    await svc.update_chapter(sid, c1["id"], stated_part=1, set_part=True)
-    await svc.update_chapter(sid, c3["id"], stated_part=3, set_part=True)
-    data = await svc.list_pages(sid)
-    assert len(data["part_gaps"]) == 1
-    gap = data["part_gaps"][0]
-    assert gap["after_chapter_id"] == c1["id"]
-    assert gap["start"] == 2
-    assert gap["end"] == 2
-
-
-@pytest.mark.asyncio
-async def test_source_post_label_when_pages_share_one_post(db):
+async def test_source_post_label_is_per_page(db):
     svc = SeriesService(db)
     artist = await _artist(db, "Src Artist")
     post = await _post(db, artist, "Source Comic pages 1-2", "pp5")
     await _post_images(db, post, artist, 2)
     out = await svc.promote_post_to_series(post.id)
     data = await svc.list_pages(out["series_tag_id"])
-    sp = data["chapters"][0]["source_post"]
+    sp = data["pages"][0]["source_post"]
     assert sp is not None
     assert sp["id"] == post.id
     assert sp["title"] == "Source Comic pages 1-2"