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/models/series_page.py

@@ -14,7 +14,7 @@ number parsed from the source post, nullable when unknown.
 
 from datetime import datetime
 
-from sqlalchemy import DateTime, ForeignKey, Integer, func
+from sqlalchemy import DateTime, ForeignKey, Integer, String, func
 from sqlalchemy.orm import Mapped, mapped_column
 
 from .base import Base
@@ -32,7 +32,12 @@ class SeriesPage(Base):
         nullable=False,
         unique=True,
     )
-    page_number: Mapped[int] = mapped_column(Integer, nullable=False)
+    # 'placed' = in the series-global run (page_number set); 'pending' = staged
+    # from a post awaiting the operator's sort (page_number NULL). (#789 P2)
+    status: Mapped[str] = mapped_column(
+        String(16), nullable=False, server_default="placed"
+    )
+    page_number: Mapped[int | None] = mapped_column(Integer, nullable=True)
     stated_page: Mapped[int | None] = mapped_column(Integer, nullable=True)
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, server_default=func.now()