Save a static QR code from structured fields

Builds the payload server-side from fields and saves it as a static code: the content is encoded directly into the QR image, with no short link and no scan analytics. Static codes cannot be edited afterwards — save a new one to change the content. Field names per type: url: url · text: text · email: email, subject?, body? · phone: phone · sms: phone, message? · whatsapp: phone (country code, digits only), message? · wifi: ssid, password?, encryption? (WPA|WEP|nopass), hidden? · geo: lat, lng · vcard: firstName?, lastName?, phone?, email?, org?, title?, url?, address? (at least one of name/phone/email). Render the returned payload as an image via /v1/qr.