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/src/stores/seriesManage.js

@@ -11,16 +11,17 @@ export function moveItem(arr, from, to) {
   return next
 }
 
+// FC-6.x: a series is ONE flat, series-global ordered run of pages, with
+// optional cosmetic chapter DIVIDERS anchored to the page that begins them.
 export const useSeriesManageStore = defineStore('seriesManage', () => {
   const api = useApi()
 
   const tagId = ref(null)
   const series = ref(null)
-  const chapters = ref([])           // [{id, chapter_number, stated_part, title, is_placeholder, stated_page_start/end, source_post, pages:[...]}]
-  const gaps = ref([])               // missing-page gaps: [{after_chapter_id, start, end}]
-  const partGaps = ref([])           // missing-Part gaps: [{after_chapter_id, start, end}]
+  const pages = ref([])              // flat, ordered: [{image_id, page_number, stated_page, thumbnail_url, image_url, source_post}]
+  const dividers = ref([])           // [{id, anchor_image_id, page_number, title, stated_part}]
+  const partGaps = ref([])           // [{after_divider_id, start, end}]
   const pageCount = ref(0)
-  const targetChapterId = ref(null)  // which chapter the picker adds into
   const picker = ref([])             // gallery scroll results
   const pickerCursor = ref(null)
   const pickerSelection = ref([])    // image ids
@@ -32,15 +33,10 @@ export const useSeriesManageStore = defineStore('seriesManage', () => {
     try {
       const body = await api.get(`/api/series/${id}/pages`)
       series.value = body.series
-      chapters.value = body.chapters || []
-      gaps.value = body.gaps || []
+      pages.value = body.pages || []
+      dividers.value = body.dividers || []
       partGaps.value = body.part_gaps || []
-      pageCount.value = (body.pages || []).length
-      // Keep a valid add-target: the selected chapter, else the first one.
-      const ids = chapters.value.map(c => c.id)
-      if (!ids.includes(targetChapterId.value)) {
-        targetChapterId.value = ids.length ? ids[0] : null
-      }
+      pageCount.value = pages.value.length
     } finally {
       loading.value = false
     }
@@ -50,79 +46,66 @@ export const useSeriesManageStore = defineStore('seriesManage', () => {
     if (tagId.value != null) await load(tagId.value)
   }
 
-  function gapAfter(chapterId) {
-    return gaps.value.find(g => g.after_chapter_id === chapterId) || null
+  // The divider anchored at a given page (shown as a chapter bar before it).
+  function dividerAt(imageId) {
+    return dividers.value.find(d => d.anchor_image_id === imageId) || null
   }
 
-  function partGapAfter(chapterId) {
-    return partGaps.value.find(g => g.after_chapter_id === chapterId) || null
+  function partGapAfter(dividerId) {
+    return partGaps.value.find(g => g.after_divider_id === dividerId) || null
   }
 
-  // ---- chapters ----
-  async function createChapter({ title = null, isPlaceholder = false } = {}) {
-    await api.post(`/api/series/${tagId.value}/chapters`, {
-      body: { title, is_placeholder: isPlaceholder }
-    })
-    await refresh()
-  }
-
-  async function renameChapter(chapterId, title) {
-    await api.patch(`/api/series/${tagId.value}/chapters/${chapterId}`, {
-      body: { title }
-    })
-    await refresh()
-  }
-
-  async function setChapterPart(chapterId, part) {
-    await api.patch(`/api/series/${tagId.value}/chapters/${chapterId}`, {
-      body: { stated_part: part }
-    })
-    await refresh()
-  }
-
-  async function setChapterStated(chapterId, start, end) {
-    await api.patch(`/api/series/${tagId.value}/chapters/${chapterId}`, {
-      body: { stated_page_start: start, stated_page_end: end }
-    })
-    await refresh()
-  }
-
-  async function reorderChapters(orderedChapterIds) {
-    await api.post(`/api/series/${tagId.value}/chapters/reorder`, {
-      body: { chapter_ids: orderedChapterIds }
-    })
-    await refresh()
-  }
-
-  async function moveChapter(chapterId, dir) {
-    const ids = chapters.value.map(c => c.id)
-    const from = ids.indexOf(chapterId)
-    const to = from + dir
-    if (from === -1 || to < 0 || to >= ids.length) return
-    await reorderChapters(moveItem(ids, from, to))
-  }
-
-  async function deleteChapter(chapterId) {
-    await api.delete(`/api/series/${tagId.value}/chapters/${chapterId}`)
-    await refresh()
-  }
-
-  async function mergeChapter(sourceId, targetId) {
-    await api.post(`/api/series/${tagId.value}/chapters/${sourceId}/merge`, {
-      body: { target_chapter_id: targetId }
-    })
-    await refresh()
-    toast({ text: 'Chapters merged', type: 'success' })
-  }
-
-  async function reorderPages(chapterId, orderedImageIds) {
-    await api.post(`/api/series/${tagId.value}/chapters/${chapterId}/reorder`, {
+  // ---- pages (series-wide) ----
+  async function reorder(orderedImageIds) {
+    await api.post(`/api/series/${tagId.value}/reorder`, {
       body: { image_ids: orderedImageIds }
     })
     await refresh()
   }
 
-  // ---- picker / pages ----
+  async function remove(imageId) {
+    await api.post(`/api/series/${tagId.value}/pages/remove`, {
+      body: { image_ids: [imageId] }
+    })
+    await refresh()
+  }
+
+  async function setCover(imageId) {
+    await api.post(`/api/series/${tagId.value}/cover`, {
+      body: { image_id: imageId }
+    })
+    await refresh()
+  }
+
+  // ---- chapter dividers ----
+  async function createDivider(anchorImageId, { title = null, statedPart = null } = {}) {
+    await api.post(`/api/series/${tagId.value}/chapters`, {
+      body: { anchor_image_id: anchorImageId, title, stated_part: statedPart }
+    })
+    await refresh()
+  }
+
+  async function renameDivider(dividerId, title) {
+    await api.patch(`/api/series/${tagId.value}/chapters/${dividerId}`, {
+      body: { title }
+    })
+    await refresh()
+  }
+
+  async function setDividerPart(dividerId, part) {
+    await api.patch(`/api/series/${tagId.value}/chapters/${dividerId}`, {
+      body: { stated_part: part }
+    })
+    await refresh()
+  }
+
+  async function deleteDivider(dividerId) {
+    await api.delete(`/api/series/${tagId.value}/chapters/${dividerId}`)
+    await refresh()
+    toast({ text: 'Chapter divider removed', type: 'success' })
+  }
+
+  // ---- picker / add pages ----
   async function loadPicker(reset = false) {
     if (reset) { picker.value = []; pickerCursor.value = null }
     const params = { limit: 50 }
@@ -140,35 +123,20 @@ export const useSeriesManageStore = defineStore('seriesManage', () => {
 
   async function addSelected() {
     if (pickerSelection.value.length === 0) return
-    if (targetChapterId.value == null) await createChapter()
     await api.post(`/api/series/${tagId.value}/pages`, {
-      body: { image_ids: pickerSelection.value, chapter_id: targetChapterId.value }
+      body: { image_ids: pickerSelection.value }
     })
     pickerSelection.value = []
     await refresh()
-    toast({ text: 'Added to chapter', type: 'success' })
-  }
-
-  async function remove(imageId) {
-    await api.post(`/api/series/${tagId.value}/pages/remove`, {
-      body: { image_ids: [imageId] }
-    })
-    await refresh()
-  }
-
-  async function setCover(imageId) {
-    await api.post(`/api/series/${tagId.value}/cover`, {
-      body: { image_id: imageId }
-    })
-    await refresh()
+    toast({ text: 'Added to series', type: 'success' })
   }
 
   return {
-    tagId, series, chapters, gaps, partGaps, pageCount, targetChapterId,
+    tagId, series, pages, dividers, partGaps, pageCount,
     picker, pickerCursor, pickerSelection, loading,
-    load, refresh, gapAfter, partGapAfter,
-    createChapter, renameChapter, setChapterPart, setChapterStated,
-    reorderChapters, moveChapter, deleteChapter, mergeChapter, reorderPages,
-    loadPicker, togglePick, addSelected, remove, setCover
+    load, refresh, dividerAt, partGapAfter,
+    reorder, remove, setCover,
+    createDivider, renameDivider, setDividerPart, deleteDivider,
+    loadPicker, togglePick, addSelected
   }
 })