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_pending.py

@@ -76,25 +76,29 @@ async def test_place_pending_appends_and_flips_status(db):
     imgs = await _post_images(db, post, artist, 3)
     await svc.add_post(sid, post.id)
 
-    placed = await svc.place_pending(sid, imgs)
+    placed = await svc.place_pending(sid, imgs, start_page=3)
     assert placed == 3
     assert await _placed_order(db, sid) == seed + imgs
-    # nothing left pending; page numbers compact 1..5
+    # nothing left pending; the placed pages are numbered 1,2 (seed) + 3,4,5
     data = await svc.list_pages(sid)
     assert data["pending"] == []
     assert [p["page_number"] for p in data["pages"]] == [1, 2, 3, 4, 5]
 
 
 @pytest.mark.asyncio
-async def test_place_pending_before_a_placed_page(db):
-    svc, artist, sid, seed = await _setup(db, "PB")
-    a, b = seed
+async def test_place_pending_numbers_from_start(db):
+    svc, artist, sid, seed = await _setup(db, "PB")  # seed numbered 1, 2
     post = await _post(db, artist, "PB pages", "pb-p")
     x, y = await _post_images(db, post, artist, 2)
     await svc.add_post(sid, post.id)
 
-    await svc.place_pending(sid, [x, y], before_image_id=b)
-    assert await _placed_order(db, sid) == [a, x, y, b]
+    await svc.place_pending(sid, [x, y], start_page=10)
+    data = await svc.list_pages(sid)
+    nums = {p["image_id"]: p["page_number"] for p in data["pages"]}
+    assert nums[x] == 10
+    assert nums[y] == 11
+    # order = seed (1, 2) then x (10), y (11)
+    assert [p["image_id"] for p in data["pages"]] == seed + [x, y]
 
 
 @pytest.mark.asyncio