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/ViewerLeaderSkinSetClaim.cs

@@ -0,0 +1,14 @@
+namespace SVSim.Database.Models;
+
+/// <summary>
+/// One row per (viewer, leader-skin series) marking that the viewer has claimed the
+/// series-completion bonus via /leader_skin/buy_set_item. Composite PK (ViewerId, SeriesId).
+/// Standalone table (not a Viewer owned collection) to avoid the cartesian-explode pitfall
+/// when loading the viewer graph — claim state is checked per-series, not per-viewer-load.
+/// </summary>
+public class ViewerLeaderSkinSetClaim
+{
+    public long ViewerId { get; set; }
+    public int SeriesId { get; set; }
+    public DateTime ClaimedAt { get; set; }
+}