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>

go.mod

@@ -76,6 +76,7 @@ require (
 	github.com/subosito/gotenv v1.4.2 // indirect
 	github.com/twitchyliquid64/golang-asm v0.15.1 // indirect
 	github.com/ugorji/go/codec v1.2.12 // indirect
+	github.com/yuin/gopher-lua v1.1.1 // indirect
 	github.com/zeebo/errs v1.4.0 // indirect
 	go.opentelemetry.io/auto/sdk v1.1.0 // indirect
 	go.opentelemetry.io/contrib/detectors/gcp v1.35.0 // indirect
@@ -107,6 +108,7 @@ require (
 
 require (
 	firebase.google.com/go/v4 v4.19.0
+	github.com/alicebob/miniredis/v2 v2.39.0
 	github.com/aws/aws-sdk-go v1.55.7
 	github.com/boombuler/barcode v1.1.0
 	github.com/golang-jwt/jwt/v5 v5.2.3