feat: add Redis support for distributed passkey sessions

- Implement SessionStore interface for pluggable storage backends
- Add redisStore for distributed environment
- Add memoryStore as fallback for single instance
- Update router to initialize Redis client when configured
- Update .env.example with Redis documentation

backend/router/setRouter.go

@@ -22,6 +22,7 @@ import (
 	"time"
 
 	"github.com/gin-gonic/gin"
+	"github.com/redis/go-redis/v9"
 	"gorm.io/gorm"
 )
 
@@ -64,10 +65,19 @@ func SetRouter(cfg *config.Config, db *gorm.DB, web *embed.FS) {
 	gateway.SetUsageRecorder(usageRecorder)
 
 	// Initialize passkey service
+	var rdb *redis.Client
+	if cfg.RedisHost != "" {
+		rdb = redis.NewClient(&redis.Options{
+			Addr:     fmt.Sprintf("%s:%d", cfg.RedisHost, cfg.RedisPort),
+			Password: cfg.RedisPassword,
+			DB:       cfg.RedisDB,
+		})
+	}
 	passkeySvc, err := passkey.New(db, passkey.Config{
 		RPID:    cfg.RPID,
 		Origins: cfg.RPOrigins,
 		Name:    cfg.AppName,
+		Redis:   rdb,
 	})
 	if err != nil {
 		log.Fatalf("Failed to initialize passkey service: %v", err)