fix: Redis集群查询+metrics修复 — 单节点连接替代ClusterClient

- crypto.go: decryptPassword 自动处理 enc: 前缀，避免调用方遗漏
- redis.go: getRedisClient 统一使用 redis.NewClient 单节点连接
  ClusterClient 拓扑发现(CLUSTER SLOTS)在Docker内网IP不可达时超时
  redisClientWrapper 全部方法改为使用 Client (不再走 ClusterClient)
  ScanAllKeys 简化为单节点 SCAN
- connection_tester.go: getRedisMetrics 统一单节点连接采集metrics
  删除 isCluster 分支的 ClusterClient 代码，避免5秒超时

backend/internal/handler/crypto.go

@@ -6,6 +6,7 @@ import (
 	"crypto/rand"
 	"encoding/base64"
 	"os"
+	"strings"
 )
 
 // getSecretKey returns the AES key derived from SECRET_KEY env var.
@@ -26,8 +27,12 @@ func getSecretKey() []byte {
 
 // decryptPassword decrypts a base64-encoded AES-256-GCM ciphertext.
 // Mirrors Python's crypto.decrypt_password().
-// If the value is not valid encrypted data (e.g., plaintext), returns it as-is.
+// Handles "enc:" prefix automatically. If not valid encrypted data, returns as-is.
 func decryptPassword(encrypted string) (string, error) {
+	// Strip "enc:" prefix if present
+	if strings.HasPrefix(encrypted, "enc:") {
+		encrypted = strings.TrimPrefix(encrypted, "enc:")
+	}
 	raw, err := base64.StdEncoding.DecodeString(encrypted)
 	if err != nil {
 		return encrypted, nil // not valid base64 — plaintext