Set CRM order status to PACKED after Checkbox accepts the receipt

- ExoCrmClient.set_status: live SetStatus needs {Orders: [id], Status} and
  replies per order, unlike the documented {ID, Status}
- receipts.crm_status_set_at (migration 0006); set right after creation,
  retried by cron, row-locked to avoid a repeat PACKED overwriting a newer status
- CRM errors under capitalized 'Errors' and non-JSON replies are reported

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

CLAUDE.md

@@ -125,4 +125,5 @@ Stages 1–4 (scaffolding, auth/audit, CRM order queue, Nova Poshta tracking) ar
 - We never fiscalize ourselves: the cashier creates an **ETTN receipt** in Checkbox bound to a Nova Poshta TTN with payment control; Checkbox fiscalizes it when the customer pays at the NP branch. Invariant: `order total − prepayment == np_cod_amount_kopecks`. Prepayment goes into the receipt as a plain `DISCOUNT` («Знижка»), **not** `PRE_PAYMENT` — the live API rejects `PRE_PAYMENT` on ETTN with 400 `third_party.generic`. The live API also returns statuses lowercase (`EttnOut` upper-cases them).
 - 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"}}`.
 - 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()`.