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/InventoryLoadConfig.cs

@@ -0,0 +1,31 @@
+using System.Linq.Expressions;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Query;
+using SVSim.Database.Models;
+
+namespace SVSim.Database.Services.Inventory;
+
+/// <summary>
+/// Caller-supplied extra <c>.Include</c> chains on top of the canonical viewer-inventory query
+/// in <see cref="IInventoryService.BeginAsync"/>. Use to bring in extra collections needed by
+/// the calling controller (e.g. <c>MissionData</c>, <c>BuildDeckPurchases</c>).
+/// </summary>
+public sealed class InventoryLoadConfig
+{
+    internal List<Func<IQueryable<Viewer>, IQueryable<Viewer>>> Includes { get; } = new();
+
+    public InventoryLoadConfig WithInclude<TProperty>(
+        Expression<Func<Viewer, TProperty>> path)
+    {
+        Includes.Add(q => q.Include(path));
+        return this;
+    }
+
+    public InventoryLoadConfig WithInclude<TProperty, TThen>(
+        Expression<Func<Viewer, IEnumerable<TProperty>>> collectionPath,
+        Expression<Func<TProperty, TThen>> thenPath)
+    {
+        Includes.Add(q => q.Include(collectionPath).ThenInclude(thenPath));
+        return this;
+    }
+}