Retry ETTN creation on Nova Poshta rate limit instead of failing

Checkbox relays Nova Poshta's "To many requests" (20000401501) as a 4xx
third_party.generic error, not a 429, so bursts of receipt requests ended
up as failed receipts. Such responses are now CheckboxRateLimitedError:
the receipt stays pending and the worker job is retried with arq.Retry
after the "Try again after N seconds" delay plus backoff. NP timeouts
relayed the same way are treated as unavailable (unknown outcome).

The HTTP client also sends Checkbox requests one at a time with a
CHECKBOX_MIN_REQUEST_INTERVAL_MS pause and signs the cashier in once for
concurrent jobs.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/tests/test_receipts_service.py

@@ -18,7 +18,7 @@ from app.db.models.order import Order
 from app.db.models.receipt import Receipt, ReceiptStatus
 from app.schemas.checkbox import EttnStatus
 from app.services import receipts as svc
-from app.services.checkbox.client import CheckboxUnavailableError
+from app.services.checkbox.client import CheckboxRateLimitedError, CheckboxUnavailableError
 from app.services.checkbox.stub_client import StubCheckboxClient
 
 
@@ -266,6 +266,24 @@ class TestCreateEttn:
         assert receipt.status == ReceiptStatus.CREATED
         assert len(client.orders) == 1  # второго чека на ту же ТТН нет
 
+    async def test_rate_limit_keeps_pending_without_unknown_outcome(self) -> None:
+        order, register = _order(receipt_created_at=datetime.now(UTC)), _register()
+        receipt = _pending(order, register)
+        client = StubCheckboxClient()
+        session = FakeSession(order, register, receipt)
+
+        async def rate_limited(creds: Any, body: dict[str, Any]) -> Any:
+            raise CheckboxRateLimitedError("To many requests", retry_after=1)
+
+        client.create_ettn = rate_limited  # type: ignore[method-assign]
+        with pytest.raises(CheckboxRateLimitedError):
+            await svc.create_ettn_for_receipt(session, client, receipt.id)
+
+        assert receipt.status == ReceiptStatus.PENDING
+        assert receipt.error is None  # повтор не пойдёт через find_ettn
+        assert order.receipt_created_at is not None  # заказ не вернулся в очередь
+        assert session.commits == 1  # блокировка строки снята
+
 
 class TestApplyEttn:
     @pytest.mark.parametrize(