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/connection_tester.go

@@ -42,7 +42,7 @@ func testDBConnection(instance *model.Instance) (bool, string) {
 		}
 		return testPostgreSQL(ctx, instance.Host, instance.Port, instance.Username, password, dbname)
 	case "redis":
-		ok, msg, _ := testRedis(ctx, instance.Host, instance.Port, password)
+		ok, msg, _ := testRedis(ctx, instance.Host, instance.Port, instance.Username, password)
 		return ok, msg
 	default:
 		return false, fmt.Sprintf("Unsupported database type: %s", instance.DBType)
@@ -71,7 +71,7 @@ func testDBConnectionFull(instance *model.Instance) *ConnectionResult {
 		ok, msg := testPostgreSQL(ctx, instance.Host, instance.Port, instance.Username, password, dbname)
 		return &ConnectionResult{Success: ok, Message: msg, Version: extractVersion(msg, "version: ")}
 	case "redis":
-		ok, msg, isCluster := testRedis(ctx, instance.Host, instance.Port, password)
+		ok, msg, isCluster := testRedis(ctx, instance.Host, instance.Port, instance.Username, password)
 		return &ConnectionResult{Success: ok, Message: msg, Version: extractVersion(msg, "version: "), ClusterMode: isCluster}
 	default:
 		return &ConnectionResult{Success: false, Message: fmt.Sprintf("Unsupported database type: %s", instance.DBType)}
@@ -132,10 +132,16 @@ func testPostgreSQL(ctx context.Context, host string, port int, user, pass, dbna
 	return true, fmt.Sprintf("PostgreSQL connection successful (version: %s)", ver)
 }
 
-func testRedis(ctx context.Context, host string, port int, pass string) (bool, string, bool) {
+func testRedis(ctx context.Context, host string, port int, username, pass string) (bool, string, bool) {
 	addr := fmt.Sprintf("%s:%d", host, port)
+	// Redis 6.0+ ACL: empty username = traditional password-only auth
+	redisUsername := username
+	if redisUsername == "default" {
+		redisUsername = ""
+	}
 	rdb := redis.NewClient(&redis.Options{
 		Addr:         addr,
+		Username:     redisUsername,
 		Password:     pass,
 		DB:           0,
 		DialTimeout:  10 * time.Second,
@@ -199,11 +205,17 @@ func getRedisMetrics(instance *model.Instance) map[string]interface{} {
 		"cluster_mode": isCluster,
 	}
 
+	// Redis 6.0+ ACL support
+	redisUsername := instance.Username
+	if redisUsername == "default" {
+		redisUsername = ""
+	}
 	var info string
 	// For metrics collection, always use single-node connection even for clusters.
 	// ClusterClient does topology discovery which can timeout if some nodes are unreachable.
 	rdb := redis.NewClient(&redis.Options{
 		Addr:         addr,
+		Username:     redisUsername,
 		Password:     password,
 		DB:           0,
 		DialTimeout:  5 * time.Second,