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.UnitTests/Infrastructure/SVSimTestFactory.cs

@@ -210,6 +210,7 @@ internal sealed class SVSimTestFactory : WebApplicationFactory<Program>
         await new PaymentItemImporter().ImportAsync(ctx, seedDir);
         await new ItemImporter().ImportAsync(ctx, seedDir);
         await new SleeveShopImporter().ImportAsync(ctx, seedDir);
+        await new ItemPurchaseImporter().ImportAsync(ctx, seedDir);
         var puzzleImporter = new PuzzleImporter();
         await puzzleImporter.ImportGroupsAsync(ctx, seedDir);
         await puzzleImporter.ImportPuzzlesAsync(ctx, seedDir);