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/SVSimDbContext.cs

@@ -72,6 +72,7 @@ public class SVSimDbContext : DbContext
     public DbSet<BuildDeckProductEntry> BuildDeckProducts => Set<BuildDeckProductEntry>();
     public DbSet<SleeveShopSeriesEntry> SleeveShopSeries => Set<SleeveShopSeriesEntry>();
     public DbSet<SleeveShopProductEntry> SleeveShopProducts => Set<SleeveShopProductEntry>();
+    public DbSet<ItemPurchaseCatalogEntry> ItemPurchaseCatalog => Set<ItemPurchaseCatalogEntry>();
     public DbSet<MaintenanceCardEntry> MaintenanceCards => Set<MaintenanceCardEntry>();
     public DbSet<FeatureMaintenanceEntry> FeatureMaintenances => Set<FeatureMaintenanceEntry>();
     public DbSet<PreReleaseInfo> PreReleaseInfos => Set<PreReleaseInfo>();