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/receipts.py

@@ -38,6 +38,7 @@ from app.services.checkbox.client import (
     CheckboxClient,
     CheckboxCredentials,
     CheckboxError,
+    CheckboxRateLimitedError,
     CheckboxUnavailableError,
 )
 from app.services.crm.client import CrmClient, CrmError
@@ -387,7 +388,10 @@ def _apply_ettn(receipt: Receipt, ettn: EttnOut) -> None:
 async def create_ettn_for_receipt(
     session: AsyncSession, client: CheckboxClient, receipt_id: uuid.UUID
 ) -> Receipt | None:
-    """Отправляет `pending`-чек в Checkbox. Идемпотентна — безопасно вызывать повторно."""
+    """Отправляет `pending`-чек в Checkbox. Идемпотентна — безопасно вызывать повторно.
+
+    На лимит частоты пробрасывает `CheckboxRateLimitedError`, чек остаётся `pending`.
+    """
     receipt = await session.get(Receipt, receipt_id, with_for_update=True)
     if receipt is None or receipt.status != ReceiptStatus.PENDING:
         return receipt
@@ -403,6 +407,12 @@ async def create_ettn_for_receipt(
             await client.find_ettn(creds, receipt.waybill_number) if receipt.error else None
         )
         ettn = existing or await client.create_ettn(creds, receipt.request_body)
+    except CheckboxRateLimitedError as exc:
+        # Запрос отклонён, чек точно не создан: остаётся `pending` без пометки
+        # «исход неизвестен». Повтор с задержкой — забота вызывающего (worker).
+        await session.commit()  # снять блокировку строки
+        log.info("ettn_rate_limited", receipt_id=str(receipt_id), retry_after=exc.retry_after)
+        raise
     except CheckboxUnavailableError as exc:
         receipt.error = str(exc)
         await session.commit()
@@ -435,7 +445,10 @@ async def retry_pending_receipts(session: AsyncSession, client: CheckboxClient)
         )
     )
     for receipt_id in receipt_ids:
-        await create_ettn_for_receipt(session, client, receipt_id)
+        try:
+            await create_ettn_for_receipt(session, client, receipt_id)
+        except CheckboxRateLimitedError:
+            break  # остальные — в следующем проходе cron'а
 
 
 # --- Отмена и опрос ----------------------------------------------------------