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>

CLAUDE.md

@@ -126,4 +126,5 @@ Stages 1–4 (scaffolding, auth/audit, CRM order queue, Nova Poshta tracking) ar
 - Two-phase create: `POST /receipts` writes `Receipt(pending)` + audit and commits, then enqueues `create_ettn_receipt`; the worker calls Checkbox. A timeout leaves the row `pending` with `error` set — the retry first looks the TTN up via `find_ettn` instead of blindly re-posting (would create a second receipt). Keep this.
 - State machine and the "one live receipt per order" partial unique index live in `app/db/models/receipt.py`. `orders.receipt_created_at` is set on request and reset to NULL when a receipt ends `failed`/`cancelled` (order goes back to the queue).
 - Once Checkbox accepts the receipt, the order is moved to `PACKED` in the CRM (`services/receipts.sync_crm_statuses`, marked by `receipts.crm_status_set_at`; runs right after creation and is retried by cron). The live exoCRM `SetStatus` differs from its docs: params must be `{"Orders": [id], "Status": ...}` (the documented `{"ID": ...}` returns "Undefined order list."), and the reply has no `status: OK` — success is `{"<id>": {"Status": "Success"}}`.
+- Nova Poshta's rate limit comes back through Checkbox as a 4xx with `code=third_party.generic` and «To many requests» / `20000401501`, not as a 429. `http_client._transient_error` maps it to `CheckboxRateLimitedError`: the receipt stays `pending` and the worker retries with `arq.Retry`. The client also sends requests one at a time with a `CHECKBOX_MIN_REQUEST_INTERVAL_MS` pause, so don't parallelize Checkbox calls in the worker.
 - ETTN does **not** work on a Checkbox test cash register. Locally use `CHECKBOX_USE_STUB=true`; client selection is only in `services/checkbox/client.get_checkbox_client()`.