[management,signal,proxy,relay,misc] Unify service configuration loading

Service entry points currently resolve defaults, files, environment variables, and flags differently, which makes precedence inconsistent and prevents some services from using config files.

Introduce one Viper-backed loader and migrate Combined, Management, Relay, Signal, and Proxy while preserving compatibility aliases and Management template expansion.

combined/cmd/root.go

@@ -56,7 +56,8 @@ var (
 All services (Management, Signal, Relay) are multiplexed on a single port.
 Optional STUN server runs on separate UDP ports.
 
-Configuration is loaded from a YAML file specified with --config.`,
+Configuration is loaded from a file specified with --config. Values can be
+overridden with NB_-prefixed environment variables, such as NB_SERVER_LOGLEVEL.`,
 		SilenceUsage:  true,
 		SilenceErrors: true,
 		RunE:          execute,
@@ -64,7 +65,7 @@ Configuration is loaded from a YAML file specified with --config.`,
 )
 
 func init() {
-	rootCmd.PersistentFlags().StringVarP(&configPath, "config", "c", "", "path to YAML configuration file (required)")
+	rootCmd.PersistentFlags().StringVarP(&configPath, "config", "c", "", "path to configuration file (required)")
 	_ = rootCmd.MarkPersistentFlagRequired("config")
 
 	rootCmd.AddCommand(newAdminCommands())