Add Nova Poshta tracking: TTN status, COD amount, payment status

Adds NpTrackingClient (Protocol + real/stub impls) and an ARQ worker that
polls Nova Poshta every minute for orders without a receipt, writing
status, net COD amount (Контроль оплати), and payment status onto the
order. Surfaced in the orders table and detail modal. Marks plan stages
3-4 done in README.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/pages/DashboardPage.css

@@ -229,6 +229,11 @@
   color: #166534;
 }
 
+.orders-status--danger {
+  background: #fee2e2;
+  color: #991b1b;
+}
+
 @media (prefers-color-scheme: dark) {
   .orders-status--processing {
     background: rgba(217, 119, 6, 0.18);
@@ -249,6 +254,11 @@
     background: rgba(22, 163, 74, 0.18);
     color: #86efac;
   }
+
+  .orders-status--danger {
+    background: rgba(220, 38, 38, 0.18);
+    color: #fca5a5;
+  }
 }
 
 .orders-actions-cell {