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>

backend/app/db/models/order.py

@@ -43,3 +43,10 @@ class Order(TimestampMixin, Base):
 
     # Заполняется будущей интеграцией с Checkbox — сейчас всегда NULL.
     receipt_created_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), index=True)
+
+    # Статус ТТН Nova Poshta — опрашивается ARQ worker'ом раз в минуту для
+    # заказов без чека, у которых есть номер ТТН (см. app/worker.py).
+    np_status: Mapped[str | None] = mapped_column(String(255))
+    np_status_code: Mapped[str | None] = mapped_column(String(16))
+    np_cod_amount_kopecks: Mapped[int | None] = mapped_column(BigInteger)
+    np_payment_status: Mapped[str | None] = mapped_column(String(32))