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>

alembic/versions/0048_series_page_pending_status.py

@@ -0,0 +1,45 @@
+"""series_page pending staging: status + nullable page_number (#789 Phase 2)
+
+Pages added from a post no longer append straight into the run — they land
+'pending' with a NULL page_number, staged grouped by their source post so the
+operator can drop junk (text-free alts, bumpers) and place the keepers into the
+sequence. A page only gets a series-global page_number once it's 'placed'.
+
+Revision ID: 0048
+Revises: 0047
+Create Date: 2026-06-11
+
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0048"
+down_revision: Union[str, None] = "0047"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "series_page",
+        sa.Column(
+            "status", sa.String(length=16), nullable=False,
+            server_default="placed",
+        ),
+    )
+    op.alter_column(
+        "series_page", "page_number",
+        existing_type=sa.Integer(), nullable=True,
+    )
+
+
+def downgrade() -> None:
+    # Lossy: pending pages are unsorted staging rows with no order — drop them.
+    op.execute("DELETE FROM series_page WHERE status = 'pending'")
+    op.alter_column(
+        "series_page", "page_number",
+        existing_type=sa.Integer(), nullable=False,
+    )
+    op.drop_column("series_page", "status")