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>

tests/test_series_match_service.py

@@ -90,8 +90,10 @@ async def test_accept_appends_pages_and_marks_added(db):
 
     await svc.accept(sug["id"])
     data = await SeriesService(db).list_pages(sid)
-    # b's pages were appended to the series' flat run (3 from a + 3 from b).
-    assert len(data["pages"]) == 6
+    # b's pages are STAGED pending (3 from a placed + 3 from b pending),
+    # awaiting the operator's sort.
+    assert len(data["pages"]) == 3
+    assert sum(len(g["pages"]) for g in data["pending"]) == 3
     status = await db.scalar(
         select(SeriesSuggestion.status).where(SeriesSuggestion.id == sug["id"])
     )