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>

backend/app/models/series_chapter.py

@@ -12,6 +12,12 @@ A chapter may be a placeholder (is_placeholder=True) — a reserved empty slot f
 a section the operator doesn't have yet; it holds no pages and shows as a gap in
 the reader. stated_page_start/end carry the page range parsed from the source
 post (FC-6.2), used to flag missing-page gaps; both are nullable when unknown.
+
+stated_part is the operator-facing "Part N" label (FC-6.4), separate from the
+positional chapter_number: chapter_number is auto-managed ordering (rewritten
+1..N on reorder/delete), while stated_part is the real installment number the
+operator types — e.g. a series authored from a post that is Part 2 of a story.
+Nullable when unset (the UI then falls back to showing chapter_number).
 """
 
 from datetime import datetime
@@ -30,6 +36,7 @@ class SeriesChapter(Base):
         ForeignKey("tag.id", ondelete="CASCADE"), nullable=False, index=True
     )
     chapter_number: Mapped[int] = mapped_column(Integer, nullable=False)
+    stated_part: Mapped[int | None] = mapped_column(Integer, nullable=True)
     title: Mapped[str | None] = mapped_column(Text, nullable=True)
     is_placeholder: Mapped[bool] = mapped_column(
         Boolean, nullable=False, server_default="false"