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/LeaderSkinShopSeriesEntry.cs

@@ -0,0 +1,33 @@
+using SVSim.Database.Common;
+
+namespace SVSim.Database.Models;
+
+/// <summary>
+/// One leader-skin-shop series (a themed collection — e.g. "7th Anniversary Skins").
+/// PK = wire series_id. <see cref="SetSalesStatus"/> controls whether the per-series
+/// "buy whole set" UI is offered: 0=none (single-skin purchases only), non-zero=set sale active.
+/// When set-active, the set-price + set-completion-reward fields are populated.
+/// </summary>
+public class LeaderSkinShopSeriesEntry : BaseEntity<int>
+{
+    public bool IsNew { get; set; }
+    public bool IsEnabled { get; set; }
+
+    /// <summary>SkinSeriesPurchaseInfo.eSetSalesStatus — 0=None.</summary>
+    public int SetSalesStatus { get; set; }
+
+    public int? SetPriceCrystal { get; set; }
+    public int? SetPriceRupy { get; set; }
+    public int? SetPriceTicket { get; set; }
+    public long? SetPriceTicketId { get; set; }
+
+    /// <summary>
+    /// SkinSeriesPurchaseInfo.RewardStatus — 0=none. The per-VIEWER claim state is computed
+    /// at request time from <see cref="ViewerLeaderSkinSetClaim"/>; this column is the catalog
+    /// default surfaced when no viewer is in context (or when set_sales_status==0).
+    /// </summary>
+    public int SetCompletionRewardStatus { get; set; }
+
+    public List<LeaderSkinShopProductEntry> Products { get; set; } = new();
+    public List<LeaderSkinShopSeriesRewardEntry> SetCompletionRewards { get; set; } = new();
+}