CUB-38: implement low filament alert logic with configurable threshold

backend/Program.cs

@@ -55,6 +55,9 @@ builder.Services.AddSingleton<IQrCodeService, QrCodeService>();
 // ── Cost Per Print Calculation ─────────────────────────────
 builder.Services.AddScoped<ICostPerPrintService, CostPerPrintService>();
 
+// ── Low Stock Detection ────────────────────────────────────
+builder.Services.AddSingleton<ILowStockDetector, LowStockDetector>();
+
 // ── FluentValidation ──────────────────────────────────────
 // Registers all validators from the API assembly into DI.
 builder.Services.AddValidatorsFromAssembly(Assembly.GetExecutingAssembly());