feat(series): operator-set sparse page numbers + gap blocks (#789 tweak)

Replaces the auto-renumbered 1..N position key with operator-OWNED page
numbers: sparse, gaps allowed, editable, never auto-renumbered. Order follows
the numbers; unnumbered pages sort to the tail. This is the fix for the model
that clobbered hand-set numbers on the flatten — numbers are now data, not a
derived sequence.

- series_service: drop the renumber-on-reorder/remove; order by page_number
  NULLS LAST; new set_page_number(image_id, n|None); list_pages returns `gaps`
  (one entry per missing-number run) + each pending group's parsed `start_page`;
  set_cover renumbers below the current min; place_pending(image_ids, start_page)
  numbers placed pages sequentially from the start (drop junk first → numbers
  line up); add_post stamps the parsed start on staged pages.
- api/tags: POST /series/<id>/pages/number (set one page's number); /pending/
  place takes start_page; removed /reorder.
- frontend: per-card editable number input; one gap block per gap with
  drop-on-edge to assign the adjacent number (middle → type); append drop zone;
  pending tray gets a "from page N" field + "Place from page N".
- tests reworked: sparse numbers + gaps, place-from-start, set-page-number route.

No migration; nothing destructive.

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

tests/test_series_service.py

@@ -84,17 +84,24 @@ async def test_remove(db):
 
 
 @pytest.mark.asyncio
-async def test_reorder_rewrites_and_rejects_mismatch(db):
+async def test_set_page_number_sparse_with_gaps(db):
     s = await TagService(db).find_or_create("S4", TagKind.series)
     i1, i2, i3 = await _img(db), await _img(db), await _img(db)
     svc = SeriesService(db)
-    await svc.add_images(s.id, [i1, i2, i3])
-    await svc.reorder(s.id, [i3, i1, i2])
-    assert await _order(db, s.id) == [i3, i1, i2]
+    await svc.add_images(s.id, [i1, i2, i3])   # numbered 1, 2, 3
+    # set a sparse number, leaving a gap
+    await svc.set_page_number(s.id, i3, 5)
+    out = await svc.list_pages(s.id)
+    assert [p["page_number"] for p in out["pages"]] == [1, 2, 5]
+    assert len(out["gaps"]) == 1
+    assert out["gaps"][0]["from"] == 3
+    assert out["gaps"][0]["to"] == 4
+    # unnumber a page → it sorts to the tail
+    await svc.set_page_number(s.id, i1, None)
+    out = await svc.list_pages(s.id)
+    assert [p["page_number"] for p in out["pages"]] == [2, 5, None]
     with pytest.raises(SeriesError):
-        await svc.reorder(s.id, [i1, i2])
-    with pytest.raises(SeriesError):
-        await svc.reorder(s.id, [i1, i2, i3, 99999])
+        await svc.set_page_number(s.id, 99999, 1)
 
 
 @pytest.mark.asyncio