fix(cluster): support external master URL

- add server.external_url / BACKUPX_SERVER_EXTERNAL_URL for Agent install URL generation
- pass the configured external Master URL into install script and compose rendering
- document cluster deployment requirements for Docker, bare-metal, and multi-node setups

Fixes #55

server/internal/app/app.go

@@ -276,7 +276,7 @@ func New(ctx context.Context, cfg config.Config, version string) (*Application,
 		UserRepository:           userRepo,
 		SystemConfigRepo:         systemConfigRepo,
 		InstallTokenService:      installTokenService,
-		MasterExternalURL:        "", // 如需覆盖 URL，可扩展 cfg.Server 增字段；目前留空依赖 X-Forwarded-* / Request.Host
+		MasterExternalURL:        cfg.Server.ExternalURL,
 		DB:                       db,
 		Metrics:                  appMetrics,
 	})