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>

backend/app/api/tags.py

@@ -583,6 +583,28 @@ async def series_add_post(tag_id: int):
     return jsonify(out)
 
 
+@tags_bp.route("/series/<int:tag_id>/pending/place", methods=["POST"])
+async def series_place_pending(tag_id: int):
+    """Move staged (pending) pages into the placed run — before `before_image_id`
+    (a placed page) or appended when omitted (#789 P2)."""
+    body = await request.get_json()
+    ids, err = _parse_bulk_ids(body, max_ids=500)
+    if err:
+        return err
+    before, berr = _opt_int(body, "before_image_id")
+    if berr:
+        return berr
+    async with get_session() as session:
+        try:
+            n = await SeriesService(session).place_pending(
+                tag_id, ids, before_image_id=before
+            )
+        except SeriesError as exc:
+            return _series_err(exc)
+        await session.commit()
+    return jsonify({"placed_count": n})
+
+
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