CUB-32: Add usage logging service with EF Core entity, service, controller, and migration

backend/Program.cs

@@ -58,6 +58,9 @@ builder.Services.AddScoped<ICostPerPrintService, CostPerPrintService>();
 // ── Low Stock Detection ────────────────────────────────────
 builder.Services.AddSingleton<ILowStockDetector, LowStockDetector>();
 
+// ── Usage Logging ───────────────────────────────────────────
+builder.Services.AddScoped<IUsageLogService, UsageLogService>();
+
 // ── FluentValidation ──────────────────────────────────────
 // Registers all validators from the API assembly into DI.
 builder.Services.AddValidatorsFromAssembly(Assembly.GetExecutingAssembly());