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.Database/Models/ItemPurchaseCatalogEntry.cs

@@ -0,0 +1,39 @@
+using SVSim.Database.Common;
+
+namespace SVSim.Database.Models;
+
+/// <summary>
+/// One row of the /item_purchase/info catalog — an exchange the user can perform N times per
+/// period (monthly or lifetime) by spending <c>RequireItem*</c> to acquire <c>PurchaseItem*</c>.
+/// PK = wire <c>purchase_id</c>.
+/// <para>
+/// Both sides reference <see cref="Enums.UserGoodsType"/>. Captures show the common shape is
+/// currency-for-item (RedEther 5000 → Seer's Globe ×1) or item-for-item (Orb Shard ×5 →
+/// Seer's Globe ×1). Per-viewer remaining quota lives in
+/// <see cref="ViewerEventCounter"/> keyed by <c>"item_purchase:{Id}"</c>.
+/// </para>
+/// </summary>
+public class ItemPurchaseCatalogEntry : BaseEntity<int>
+{
+    public int RequireItemType { get; set; }
+    public long RequireItemId { get; set; }
+    public int RequireItemNum { get; set; }
+
+    public int PurchaseItemType { get; set; }
+    public long PurchaseItemId { get; set; }
+    public int PurchaseItemNum { get; set; }
+
+    /// <summary>
+    /// SystemText-ready display name. May be empty — the client falls back to a templated name
+    /// built from <c>UserGoods.getUserGoodsName + count</c> via SystemText key "Shop_0132".
+    /// </summary>
+    public string PurchaseName { get; set; } = string.Empty;
+
+    /// <summary>True → quota resets at the start of each JST month. False → lifetime quota.</summary>
+    public bool IsMonthlyReset { get; set; }
+
+    /// <summary>Per-period purchase cap. Wire <c>rest</c> = max(0, PurchaseLimit - counter).</summary>
+    public int PurchaseLimit { get; set; }
+
+    public bool IsEnabled { get; set; }
+}