feat: add HA candidate failover to API server backend

- BackendConfig.Peers []string carries host:port candidates from YAML (backend.peers)
- RegisterToBackend iterates Peers in order, connecting to first responsive primary
- parsePeer extracted to peers.go (no build tag) so it's testable without CGO
- Single-host path (no Peers) preserved unchanged
- Tests: parsePeer table-driven (11 cases), stub RegisterToBackend with/without peers

src/frontend/apiserver/utils/config.go

@@ -13,10 +13,15 @@ import (
 )
 
 type BackendConfig struct {
-	Address string `yaml:"address"`
-	Port    int    `yaml:"port"`
-	From    string `yaml:"from"`
-	Key     string `yaml:"key"`
+	Address string   `yaml:"address"`
+	Port    int      `yaml:"port"`
+	From    string   `yaml:"from"`
+	Key     string   `yaml:"key"`
+	// Peers lists additional controller candidates for HA mode in "host:port"
+	// format.  When non-empty, RegisterToBackend iterates all candidates and
+	// connects to whichever responds as primary.  Address+Port remain the
+	// single-host fallback when Peers is empty.
+	Peers   []string `yaml:"peers"`
 }
 
 type AppServerConfig struct {
@@ -250,6 +255,7 @@ func InitConf() error {
 		Port:    viper.GetInt("backend.port"),
 		From:    viper.GetString("backend.from"),
 		Key:     viper.GetString("backend.key"),
+		Peers:   viper.GetStringSlice("backend.peers"),
 	}
 
 	return nil