feat(series): operator-set sparse page numbers + gap blocks (#789 tweak)

Replaces the auto-renumbered 1..N position key with operator-OWNED page
numbers: sparse, gaps allowed, editable, never auto-renumbered. Order follows
the numbers; unnumbered pages sort to the tail. This is the fix for the model
that clobbered hand-set numbers on the flatten — numbers are now data, not a
derived sequence.

- series_service: drop the renumber-on-reorder/remove; order by page_number
  NULLS LAST; new set_page_number(image_id, n|None); list_pages returns `gaps`
  (one entry per missing-number run) + each pending group's parsed `start_page`;
  set_cover renumbers below the current min; place_pending(image_ids, start_page)
  numbers placed pages sequentially from the start (drop junk first → numbers
  line up); add_post stamps the parsed start on staged pages.
- api/tags: POST /series/<id>/pages/number (set one page's number); /pending/
  place takes start_page; removed /reorder.
- frontend: per-card editable number input; one gap block per gap with
  drop-on-edge to assign the adjacent number (middle → type); append drop zone;
  pending tray gets a "from page N" field + "Place from page N".
- tests reworked: sparse numbers + gaps, place-from-start, set-page-number route.

No migration; nothing destructive.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/stores/seriesManage.js

@@ -20,7 +20,8 @@ export const useSeriesManageStore = defineStore('seriesManage', () => {
   const series = ref(null)
   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 pending = ref([])            // staged-from-post: [{post:{id,title}|null, pages:[{image_id, stated_page, thumbnail_url, image_url}]}]
+  const gaps = ref([])               // missing-number gaps: [{after_image_id, before_image_id, from, to}]
+  const pending = ref([])            // staged-from-post: [{post:{id,title}|null, start_page, pages:[{image_id, stated_page, thumbnail_url, image_url}]}]
   const partGaps = ref([])           // [{after_divider_id, start, end}]
   const pageCount = ref(0)
   const picker = ref([])             // gallery scroll results
@@ -36,6 +37,7 @@ export const useSeriesManageStore = defineStore('seriesManage', () => {
       series.value = body.series
       pages.value = body.pages || []
       dividers.value = body.dividers || []
+      gaps.value = body.gaps || []
       pending.value = body.pending || []
       partGaps.value = body.part_gaps || []
       pageCount.value = pages.value.length
@@ -57,14 +59,19 @@ export const useSeriesManageStore = defineStore('seriesManage', () => {
     return partGaps.value.find(g => g.after_divider_id === dividerId) || null
   }
 
-  // ---- pages (series-wide) ----
-  async function reorder(orderedImageIds) {
-    await api.post(`/api/series/${tagId.value}/reorder`, {
-      body: { image_ids: orderedImageIds }
+  // ---- pages (number-driven) ----
+  // Set one page's number (sparse, gaps allowed; pass null to unnumber).
+  async function setPageNumber(imageId, pageNumber) {
+    await api.post(`/api/series/${tagId.value}/pages/number`, {
+      body: { image_id: imageId, page_number: pageNumber }
     })
     await refresh()
   }
 
+  function gapAfter(imageId) {
+    return gaps.value.find(g => g.after_image_id === imageId) || null
+  }
+
   async function remove(imageId) {
     await api.post(`/api/series/${tagId.value}/pages/remove`, {
       body: { image_ids: [imageId] }
@@ -108,9 +115,10 @@ export const useSeriesManageStore = defineStore('seriesManage', () => {
   }
 
   // ---- pending staging (add-from-post) ----
-  async function placePending(imageIds, beforeImageId = null) {
+  // Place pending pages, numbered sequentially from startPage in the given order.
+  async function placePending(imageIds, startPage = null) {
     await api.post(`/api/series/${tagId.value}/pending/place`, {
-      body: { image_ids: imageIds, before_image_id: beforeImageId }
+      body: { image_ids: imageIds, start_page: startPage }
     })
     await refresh()
     toast({ text: 'Pages placed into the series', type: 'success' })
@@ -143,10 +151,10 @@ export const useSeriesManageStore = defineStore('seriesManage', () => {
   }
 
   return {
-    tagId, series, pages, dividers, pending, partGaps, pageCount,
+    tagId, series, pages, dividers, gaps, pending, partGaps, pageCount,
     picker, pickerCursor, pickerSelection, loading,
-    load, refresh, dividerAt, partGapAfter,
-    reorder, remove, setCover, placePending,
+    load, refresh, dividerAt, partGapAfter, gapAfter,
+    setPageNumber, remove, setCover, placePending,
     createDivider, renameDivider, setDividerPart, deleteDivider,
     loadPicker, togglePick, addSelected
   }