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/core/config.py

@@ -69,6 +69,9 @@ class Settings(BaseSettings):
     checkbox_base_url: str = "https://api.checkbox.ua"
     checkbox_client_name: str = "lux_fiscal"
     checkbox_client_version: str = "0.1.0"
+    # Пауза между запросами к Checkbox из одного процесса: при создании ЕТТН
+    # Checkbox ходит в API Новой Почты, а та отвечает «To many requests» на частые вызовы.
+    checkbox_min_request_interval_ms: int = 1000
     # Стаб вместо реального Checkbox: ЕТТН работает только на боевой кассе,
     # поэтому локально весь цикл прогоняется через стаб. В проде запрещено.
     checkbox_use_stub: bool = False