feat(services): declare entitlements + currency-spend primitives

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.Database/Services/ICurrencySpendService.cs

@@ -0,0 +1,14 @@
+using SVSim.Database.Models;
+
+namespace SVSim.Database.Services;
+
+/// <summary>
+/// Centralized debit primitive — the symmetric twin of <c>RewardGrantService.ApplyAsync</c>.
+/// Encapsulates the affordability-check + deduction + post-state-total pattern that was inlined
+/// across the shop/pack controllers. Does NOT call <c>SaveChangesAsync</c>; the caller saves.
+/// Freeplay (for Crystal/Rupee/RedEther) makes spends always succeed without deducting.
+/// </summary>
+public interface ICurrencySpendService
+{
+    Task<SpendResult> TrySpendAsync(Viewer viewer, SpendCurrency currency, long cost, CancellationToken ct = default);
+}