fix: 数据源编辑密码保留 + Redis ACL 认证支持

后端:
- UpdateInstance: 空密码和'******'都不更新密码字段，防止编辑后丢失密码
- testRedis/getRedisClient/getRedisMetrics: 增加 Username 参数支持 Redis 6.0+ ACL 认证
- username 为 'default' 或空时走传统密码认证模式

前端:
- EditInstanceDialog: Redis 用户名字段提示'6.0+ ACL 可选'，密码 placeholder 改为'不修改请留空'
- CreateInstanceForm: Redis 类型也显示用户名输入框（可选），创建/测试连接时 Redis username 为空则不传
- Redis 提示文案从'无需用户名'改为'6.0+ ACL 支持用户名+密码认证'

backend/internal/handler/redis.go

@@ -156,12 +156,19 @@ func getRedisClient(instance *model.Instance, db int) (*redisClientWrapper, erro
 	}
 	addr := fmt.Sprintf("%s:%d", instance.Host, instance.Port)
 
+	// Redis 6.0+ ACL: username is optional (empty = use "default" traditional auth)
+	redisUsername := instance.Username
+	if redisUsername == "default" || redisUsername == "" {
+		redisUsername = "" // go-redis treats empty Username as no ACL auth
+	}
+
 	// Always use single-node connection for query execution.
 	// ClusterClient does topology discovery (CLUSTER SLOTS) which fails when
 	// internal cluster IPs are unreachable from the client (e.g., Docker networks).
 	// Single-node mode works for all read-only commands on cluster nodes.
 	client := redis.NewClient(&redis.Options{
 		Addr:     addr,
+		Username: redisUsername,
 		Password: password,
 		DB:       db,
 	})