feat(inventory): scaffold InventoryService namespace types

Empty interfaces + records for IInventoryService, IInventoryTransaction,
InventoryCommitResult, InventoryLoadConfig, InventoryCatalogException.
Implementation lands in subsequent commits.

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

SVSim.Database/Services/Inventory/IInventoryTransaction.cs

@@ -0,0 +1,30 @@
+using SVSim.Database.Enums;
+using SVSim.Database.Models;
+using SVSim.Database.Services;
+
+namespace SVSim.Database.Services.Inventory;
+
+/// <summary>
+/// Scoped builder returned by <see cref="IInventoryService.BeginAsync"/>. Queue spend +
+/// grant operations; commit to save and assemble the <see cref="InventoryCommitResult"/>.
+/// <para>
+/// Dispose without committing rolls back the underlying DB transaction and detaches any
+/// in-memory mutations. <b>Always</b> wrap in <c>await using</c>.
+/// </para>
+/// </summary>
+public interface IInventoryTransaction : IAsyncDisposable
+{
+    Viewer Viewer { get; }
+    bool IsFreeplay { get; }
+
+    Task<SpendResult> TrySpendAsync(SpendCurrency currency, long cost, CancellationToken ct = default);
+    Task<SpendResult> TryDebitAsync(UserGoodsType type, long detailId, int num, CancellationToken ct = default);
+    Task<IReadOnlyList<GrantedReward>> GrantAsync(UserGoodsType type, long detailId, int num, CancellationToken ct = default);
+    Task<int> BackfillCardCosmeticsAsync(CancellationToken ct = default);
+
+    long EffectiveBalance(SpendCurrency currency);
+    bool OwnsCard(long cardId);
+    bool OwnsCosmetic(CosmeticType type, int id);
+
+    Task<InventoryCommitResult> CommitAsync(CancellationToken ct = default);
+}