feat(series): manage-view redesign — big pages, editable Part #, slide-over picker (FC-6.4)

Operator feedback: thumbnails too small to judge order, no obvious way to mark
'this installment is Part 2', and the permanent two-pane picker was busy and
competed with the ordering work.

- Full-width parts, each a card with a big page grid (150px, contain so whole
  pages are visible) and drag-to-reorder; positional page number as a badge.
- Editable Part # (hero field) backed by new series_chapter.stated_part —
  separate from the auto-managed chapter_number, mirroring the page_number vs
  stated_page split so reorder/delete renumbering can't wipe a hand-set part.
  Missing-Part hints when consecutive parts' stated_part jump >1.
- Each part labels its source post (derived from pages' primary_post_id) and
  shows the printed-page range with clear labels.
- Picker demoted to an on-demand right slide-over ('Add pages') with a target-
  part selector; part actions (move/merge/delete) collapsed into an overflow ⋮.

alembic 0042 adds series_chapter.stated_part (nullable int).

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

alembic/versions/0042_series_chapter_stated_part.py

@@ -0,0 +1,32 @@
+"""series chapter stated_part: operator-facing Part N label (FC-6.4)
+
+Revision ID: 0042
+Revises: 0041
+Create Date: 2026-06-07
+
+A chapter's positional chapter_number is auto-managed (rewritten 1..N on
+reorder/delete), so it can't double as the installment number the operator wants
+to type (e.g. a series authored from a post that is Part 2). Add a nullable
+stated_part alongside it — the same split as series_page.page_number (order) vs
+series_page.stated_page (printed number). Nullable; the UI falls back to
+chapter_number when unset.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0042"
+down_revision: Union[str, None] = "0041"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "series_chapter", sa.Column("stated_part", sa.Integer, nullable=True)
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("series_chapter", "stated_part")