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/cmd/main.go

@@ -66,6 +66,7 @@ func main() {
 			Port:    utils.BackendYamlConfig.Port,
 			From:    utils.BackendYamlConfig.From,
 			Key:     utils.BackendYamlConfig.Key,
+			Peers:   utils.BackendYamlConfig.Peers,
 		}
 		if err := backend.RegisterToBackend(bcfg); err != nil {
 			logger.Error("Failed to connect to backend", zap.Error(err))