feat(leader-skin): shop catalog + 5 endpoints (/products, /buy, /buy_set, /buy_set_item, /ids)

Schema: LeaderSkinShopSeries -> Products (owned rewards) + owned
SetCompletionRewards on the series; ViewerLeaderSkinSetClaim composite
PK (ViewerId, SeriesId) backs the /buy_set_item idempotent-claim check.

Importer mirrors SleeveShopImporter: idempotent find-or-create, owned
collections rewritten wholesale on rerun. 16 series, 104 products.

Controller (extends existing /set with 5 new endpoints):
- /products: dict-keyed-by-series_id-string wire shape. is_completed
  per-viewer, rewards.status from ViewerLeaderSkinSetClaim (0=no set
  sale, 1=available, 2=claimed) matching client RewardStatus enum.
- /buy: single skin, sales_type 1/2 dispatch, 3=>501.
- /buy_set: whole series at SetPrice; requires set_sales_status != 0;
  grants every product's rewards (RewardGrantService idempotent on
  already-owned cosmetics, so partial-set buys don't double-add).
- /buy_set_item: requires viewer owns every skin in series; idempotent
  on re-claim (returns 200 + empty reward_list, not 400) so client
  retries don't error.
- /ids: flat owned-skin-id list for badge refresh.

496 tests pass (was 486; +10 leader-skin-shop tests).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.Database/Models/LeaderSkinShopSeriesRewardEntry.cs

@@ -0,0 +1,18 @@
+using Microsoft.EntityFrameworkCore;
+
+namespace SVSim.Database.Models;
+
+/// <summary>
+/// One set-completion bonus item attached to a leader-skin series. Owned by
+/// <see cref="LeaderSkinShopSeriesEntry"/>. Granted by /leader_skin/buy_set_item once the
+/// viewer owns every skin in the series. Wire shape: entries inside
+/// <c>rewards.items[]</c> on the per-series block of /leader_skin/products.
+/// </summary>
+[Owned]
+public class LeaderSkinShopSeriesRewardEntry
+{
+    public int OrderIndex { get; set; }
+    public int RewardType { get; set; }
+    public long RewardDetailId { get; set; }
+    public int RewardNumber { get; set; }
+}