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/app/services/checkbox/client.py

@@ -23,6 +23,18 @@ class CheckboxUnavailableError(CheckboxError):
     """Сеть, таймаут или 5xx — исход запроса неизвестен, можно повторить."""
 
 
+class CheckboxRateLimitedError(CheckboxUnavailableError):
+    """Лимит частоты запросов (Checkbox 429 или «To many requests» от Новой Почты).
+
+    В отличие от прочих `CheckboxUnavailableError` исход известен — запрос отклонён,
+    повторять можно не раньше чем через `retry_after` секунд.
+    """
+
+    def __init__(self, message: str, retry_after: float) -> None:
+        super().__init__(message)
+        self.retry_after = retry_after
+
+
 @dataclass(frozen=True)
 class CheckboxCredentials:
     """Расшифрованные доступы одной кассы. Живут только в памяти."""