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/worker.py

@@ -0,0 +1,37 @@
+"""ARQ worker: раз в минуту опрашивает Nova Poshta по заказам без чека.
+
+Запускается отдельным процессом: `arq app.worker.WorkerSettings`.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+from arq import cron
+from arq.connections import RedisSettings
+
+from app.core.config import settings
+from app.core.logging import configure_logging, get_logger
+from app.db.session import SessionFactory
+from app.services.nova_poshta.np_client import NpTrackingClient
+from app.services.orders import sync_np_statuses
+
+log = get_logger(__name__)
+
+
+async def startup(ctx: dict[str, Any]) -> None:
+    configure_logging()
+    ctx["np_client"] = NpTrackingClient(settings)
+    log.info("worker_starting", environment=settings.environment)
+
+
+async def poll_np_statuses(ctx: dict[str, Any]) -> None:
+    async with SessionFactory() as session:
+        await sync_np_statuses(session, ctx["np_client"])
+    log.info("np_statuses_polled")
+
+
+class WorkerSettings:
+    redis_settings = RedisSettings.from_dsn(settings.redis_url)
+    on_startup = startup
+    cron_jobs = [cron(poll_np_statuses, minute=set(range(60)), run_at_startup=True)]