feat(item-purchase): /item_purchase/{info,purchase} + catalog

Schema: ItemPurchaseCatalogEntry (single table). Per-viewer quota tracked
via existing ViewerEventCounter keyed by "item_purchase:<id>" with period
JstPeriod.MonthKey when IsMonthlyReset else AllTime.

Controller:
- /info returns catalog + per-period rest (server-computed
  max(0, PurchaseLimit - counter)) + user_card_pack_ticket_list (every
  Items.Type==2 row joined to viewer count, zeros included — client
  unconditionally UpdateItemNum's each entry).
- /purchase: sold_out check before currency check (no counter increment
  on currency failure), inline TryDebit covers RedEther/Crystal/Rupy/Item
  with post-state-total reward_list entry, grant via RewardGrantService.
  Request `rest` accepted but ignored (server counter is canonical).

Importer mirrors PaymentItemImporter shape — idempotent find-or-create,
seed-missing rows preserved. 3 entries from the prod capture.

486 tests pass (was 476; +10 item_purchase tests).

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

SVSim.EmulatedEntrypoint/Models/Dtos/Requests/ItemPurchase/ItemPurchasePurchaseRequest.cs

@@ -0,0 +1,22 @@
+using System.Text.Json.Serialization;
+using MessagePack;
+using SVSim.EmulatedEntrypoint.Models.Dtos.Requests;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.Requests.ItemPurchase;
+
+/// <summary>
+/// /item_purchase/purchase request body. <c>rest</c> is the client's locally-cached remaining
+/// quota — used as an optional optimistic-concurrency check on the server. Not authoritative;
+/// the server's own counter is canonical.
+/// </summary>
+[MessagePackObject]
+public class ItemPurchasePurchaseRequest : BaseRequest
+{
+    [JsonPropertyName("purchase_id")]
+    [Key("purchase_id")]
+    public int PurchaseId { get; set; }
+
+    [JsonPropertyName("rest")]
+    [Key("rest")]
+    public int Rest { get; set; }
+}