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/IInventoryService.cs

@@ -0,0 +1,28 @@
+using SVSim.Database.Models;
+using SVSim.Database.Services;
+
+namespace SVSim.Database.Services.Inventory;
+
+public interface IInventoryService
+{
+    /// <summary>
+    /// Loads the viewer with the canonical inventory graph (Cards.Card, Sleeves, Emblems,
+    /// LeaderSkins, Degrees, MyPageBackgrounds, Items.Item under AsSplitQuery), opens a DB
+    /// transaction, and returns a builder for queueing operations. Throws
+    /// <see cref="InventoryViewerNotFoundException"/> if the viewer does not exist.
+    /// </summary>
+    Task<IInventoryTransaction> BeginAsync(
+        long viewerId,
+        CancellationToken ct = default,
+        Action<InventoryLoadConfig>? configure = null);
+
+    Task<IReadOnlyList<OwnedCardEntry>> EffectiveOwnedCardsAsync(Viewer viewer, CancellationToken ct = default);
+    Task<EffectiveCosmetics> EffectiveCosmeticsAsync(Viewer viewer, CancellationToken ct = default);
+    long EffectiveBalance(Viewer viewer, SpendCurrency currency);
+}
+
+public sealed class InventoryViewerNotFoundException : Exception
+{
+    public InventoryViewerNotFoundException(long viewerId)
+        : base($"Viewer {viewerId} not found") { }
+}