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/alembic/versions/0003_order_np_status.py

@@ -0,0 +1,30 @@
+"""Статус ТТН Nova Poshta на заказе
+
+Revision ID: 0003
+Revises: 0002
+Create Date: 2026-09-23
+"""
+
+from __future__ import annotations
+
+from collections.abc import Sequence
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0003"
+down_revision: str | None = "0002"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    op.add_column("orders", sa.Column("np_status", sa.String(length=255), nullable=True))
+    op.add_column("orders", sa.Column("np_status_code", sa.String(length=16), nullable=True))
+    op.add_column("orders", sa.Column("np_cod_amount_kopecks", sa.BigInteger(), nullable=True))
+
+
+def downgrade() -> None:
+    op.drop_column("orders", "np_cod_amount_kopecks")
+    op.drop_column("orders", "np_status_code")
+    op.drop_column("orders", "np_status")