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>

frontend/test/seriesManage.spec.js

@@ -13,17 +13,14 @@ function stubFetch(handler) {
   })
 }
 
+// FC-6.x: a flat page run + cosmetic chapter dividers.
 const SERIES_BODY = {
   series: { id: 7, name: 'V' },
-  chapters: [
-    {
-      id: 1, chapter_number: 1, title: null, is_placeholder: false,
-      stated_page_start: null, stated_page_end: null,
-      pages: [{ image_id: 1, page_number: 1, thumbnail_url: 't' }]
-    }
+  pages: [{ image_id: 1, page_number: 1, thumbnail_url: 't', source_post: null }],
+  dividers: [
+    { id: 1, anchor_image_id: 1, page_number: 1, title: null, stated_part: null }
   ],
-  gaps: [],
-  pages: [{ image_id: 1, page_number: 1, thumbnail_url: 't' }]
+  part_gaps: []
 }
 
 describe('seriesManage', () => {
@@ -35,17 +32,17 @@ describe('seriesManage', () => {
     expect(moveItem([1, 2, 3], 0, 2)).toEqual([2, 3, 1])
   })
 
-  it('load fetches chapters + series and picks a default target', async () => {
+  it('load fetches the flat pages + dividers + series', async () => {
     const s = useSeriesManageStore()
     stubFetch(() => ({ status: 200, body: SERIES_BODY }))
     await s.load(7)
     expect(s.series).toEqual({ id: 7, name: 'V' })
-    expect(s.chapters.map(c => c.id)).toEqual([1])
+    expect(s.pages.map(p => p.image_id)).toEqual([1])
+    expect(s.dividers.map(d => d.id)).toEqual([1])
     expect(s.pageCount).toBe(1)
-    expect(s.targetChapterId).toBe(1)
   })
 
-  it('reorderPages posts ordered ids to the chapter reorder route', async () => {
+  it('reorder posts ordered ids to the series reorder route', async () => {
     const s = useSeriesManageStore()
     s.tagId = 7
     const calls = []
@@ -54,28 +51,29 @@ describe('seriesManage', () => {
       if (url.includes('/reorder')) return { status: 200, body: { ok: true } }
       return { status: 200, body: SERIES_BODY }
     })
-    await s.reorderPages(3, [30, 10, 20])
-    const r = calls.find(c => c.url.includes('/chapters/3/reorder'))
-    expect(r.url).toContain('/api/series/7/chapters/3/reorder')
+    await s.reorder([30, 10, 20])
+    const r = calls.find(c => c.url.includes('/reorder'))
+    expect(r.url).toContain('/api/series/7/reorder')
     expect(r.body).toEqual({ image_ids: [30, 10, 20] })
   })
 
-  it('moveChapter reorders via the chapter id list', async () => {
+  it('createDivider posts the anchor image + labels', async () => {
     const s = useSeriesManageStore()
     s.tagId = 7
-    s.chapters = [{ id: 1 }, { id: 2 }, { id: 3 }]
     const calls = []
     stubFetch((url, init) => {
-      calls.push({ url, body: init.body ? JSON.parse(init.body) : null })
-      if (url.includes('/chapters/reorder')) return { status: 200, body: { ok: true } }
+      calls.push({ url, method: init.method, body: init.body ? JSON.parse(init.body) : null })
+      if (init.method === 'POST' && url.endsWith('/chapters'))
+        return { status: 200, body: { id: 2, anchor_image_id: 5 } }
       return { status: 200, body: SERIES_BODY }
     })
-    await s.moveChapter(2, -1)
-    const r = calls.find(c => c.url.includes('/chapters/reorder'))
-    expect(r.body).toEqual({ chapter_ids: [2, 1, 3] })
+    await s.createDivider(5, { title: 'X', statedPart: 2 })
+    const c = calls.find(x => x.method === 'POST' && x.url.endsWith('/chapters'))
+    expect(c.url).toContain('/api/series/7/chapters')
+    expect(c.body).toEqual({ anchor_image_id: 5, title: 'X', stated_part: 2 })
   })
 
-  it('setChapterPart patches stated_part on the chapter', async () => {
+  it('setDividerPart patches stated_part on the divider', async () => {
     const s = useSeriesManageStore()
     s.tagId = 7
     const calls = []
@@ -84,7 +82,7 @@ describe('seriesManage', () => {
       if (init.method === 'PATCH') return { status: 200, body: { ok: true } }
       return { status: 200, body: SERIES_BODY }
     })
-    await s.setChapterPart(1, 2)
+    await s.setDividerPart(1, 2)
     const p = calls.find(c => c.method === 'PATCH')
     expect(p.url).toContain('/api/series/7/chapters/1')
     expect(p.body).toEqual({ stated_part: 2 })
@@ -94,18 +92,25 @@ describe('seriesManage', () => {
     const s = useSeriesManageStore()
     stubFetch(() => ({
       status: 200,
-      body: { ...SERIES_BODY, part_gaps: [{ after_chapter_id: 1, start: 2, end: 2 }] }
+      body: { ...SERIES_BODY, part_gaps: [{ after_divider_id: 1, start: 2, end: 2 }] }
     }))
     await s.load(7)
     expect(s.partGaps).toHaveLength(1)
-    expect(s.partGapAfter(1)).toEqual({ after_chapter_id: 1, start: 2, end: 2 })
+    expect(s.partGapAfter(1)).toEqual({ after_divider_id: 1, start: 2, end: 2 })
     expect(s.partGapAfter(99)).toBeNull()
   })
 
-  it('addSelected posts selection + target chapter then clears', async () => {
+  it('dividerAt finds a divider by its anchor image', async () => {
+    const s = useSeriesManageStore()
+    stubFetch(() => ({ status: 200, body: SERIES_BODY }))
+    await s.load(7)
+    expect(s.dividerAt(1)?.id).toBe(1)
+    expect(s.dividerAt(99)).toBeNull()
+  })
+
+  it('addSelected posts the selection (no chapter) then clears', async () => {
     const s = useSeriesManageStore()
     s.tagId = 7
-    s.targetChapterId = 5
     s.pickerSelection = [9, 10]
     const calls = []
     stubFetch((url, init) => {
@@ -116,7 +121,7 @@ describe('seriesManage', () => {
     })
     await s.addSelected()
     const add = calls.find(c => c.url.endsWith('/api/series/7/pages'))
-    expect(add.body).toEqual({ image_ids: [9, 10], chapter_id: 5 })
+    expect(add.body).toEqual({ image_ids: [9, 10] })
     expect(s.pickerSelection).toEqual([])
   })
 })