feat(series): pending staging for add-from-post (#789 Phase 2)

Add-from-post no longer appends straight into the run — it STAGES the post's
pages as pending (per-page status; page_number NULL), grouped by source post,
so the operator drops junk (text-free alts, bumpers) and places the keepers
into the sequence with clean series-global numbering.

- migration 0048: series_page.status ('placed' default | 'pending') + nullable
  page_number.
- series_service: placed/pending split everywhere (list_pages returns the
  placed run + a `pending` section grouped by source post; reorder/cover/
  list_series operate on placed only); add_post stages pending; new
  place_pending(image_ids, before_image_id=None) flips pending→placed spliced
  before a page (or appended) and renumbers; junk removal reuses remove_images.
- api/tags: /add-post now returns staged count; new POST /series/<id>/pending/
  place.
- frontend: PostSeriesMenu navigates to the series after staging; seriesManage
  store surfaces `pending` + placePending; SeriesManageView gains a pending
  tray (per-post groups, place-all / place-one / drop-junk).
- tests: pending staging, place (append + insert-before), ignore-already-
  placed, drop-junk, route guard; updated add_post + match-accept expectations.

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

frontend/src/components/posts/PostSeriesMenu.vue

@@ -123,7 +123,9 @@ async function onAddExisting() {
       body: { post_id: props.post.id }
     })
     pickerOpen.value = false
-    toast({ text: 'Added to series', type: 'success' })
+    toast({ text: 'Staged — sort the new pages into the series', type: 'success' })
+    // Take the operator to the series so they can drop junk + place the pages.
+    router.push({ name: 'series-manage', params: { tagId: picked.value } })
   } catch (e) {
     toast({ text: `Could not add to series: ${e.message}`, type: 'error' })
   } finally {