feat(loyalty): one-time EnakPoint payment code

Adds POST /customer/wallet/payment-code (docs/prd-point-coin.md F9, K8,
PC-304). The customer approves with their PIN on their own phone and gets a
6-digit code, as digits and as a QR payload (enakpoint:<code>) for the app
to render, valid for two minutes. The PIN is never typed at the cashier.

Codes are drawn from crypto/rand and stored in Redis with SET NX and a TTL,
bound to the customer; a new code retires the previous one. Redeeming is a
single Lua step that uses the code up only if it belongs to the order's
customer, so it stays one-time under a race, and a cashier scanning it
against the wrong order does not burn it for its owner, which a plain
GETDEL would. Expired, used, unknown and other customers' codes are all
refused alike.

Tests run against miniredis, added as a test dependency.

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

internal/models/customer_pin.go

@@ -37,3 +37,11 @@ type CustomerPinRequestInfo struct {
 	IPAddress string
 	UserAgent string
 }
+
+// PaymentCode is what POST /customer/wallet/payment-code returns: a one-time code the
+// customer shows the cashier, as digits or as a QR of QRPayload.
+type PaymentCode struct {
+	Code      string    `json:"code"`
+	QRPayload string    `json:"qr_payload"`
+	ExpiresAt time.Time `json:"expires_at"`
+}