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/InventoryCatalogException.cs

@@ -0,0 +1,10 @@
+namespace SVSim.Database.Services.Inventory;
+
+/// <summary>
+/// Thrown when an inventory operation references a catalog id that doesn't exist
+/// (unknown card / item / cosmetic). Programmer error — bubbles to the global error handler.
+/// </summary>
+public sealed class InventoryCatalogException : Exception
+{
+    public InventoryCatalogException(string message) : base(message) { }
+}