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/api/tags.py

@@ -528,6 +528,11 @@ async def series_chapter_update(tag_id: int, chapter_id: int):
         if body["title"] is not None and not isinstance(body["title"], str):
             return jsonify({"error": "title must be a string"}), 400
         kwargs.update(set_title=True, title=body["title"])
+    if "stated_part" in body:
+        part, perr = _opt_int(body, "stated_part")
+        if perr:
+            return perr
+        kwargs.update(set_part=True, stated_part=part)
     if "stated_page_start" in body:
         start, serr = _opt_int(body, "stated_page_start")
         if serr: