Retrieve a QR code for an order

Returns a PNG image encoding TRYBE_ORDER_{orderId} as a QR code. The Trybe-prefixed payload lets the Trybe operator app distinguish order codes from other QR payloads it scans, and avoids decoding ambiguity with raw IDs.

The response is cached for one hour (Cache-Control: public, max-age=3600) — the encoded payload doesn't change for the lifetime of the order.

No authentication is required for this endpoint, matching the behaviour of print-pos-receipt and the other operator-facing print routes; the order ID is the secret.