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/config.example.yaml

@@ -3,6 +3,7 @@ server:
   host: "0.0.0.0"
   port: 8340
   mode: "release"              # debug | release
+  external_url: ""             # 可选：Master 对 Agent 可达的 URL，例如 https://backup.example.com
 
 database:
   path: "./data/backupx.db"    # SQLite 数据库路径

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,
 	})

server/internal/config/config.go

@@ -17,9 +17,10 @@ type Config struct {
 }
 
 type ServerConfig struct {
-	Host string `mapstructure:"host"`
-	Port int    `mapstructure:"port"`
-	Mode string `mapstructure:"mode"`
+	Host        string `mapstructure:"host"`
+	Port        int    `mapstructure:"port"`
+	Mode        string `mapstructure:"mode"`
+	ExternalURL string `mapstructure:"external_url"`
 }
 
 type DatabaseConfig struct {
@@ -136,6 +137,7 @@ func applyDefaults(v *viper.Viper) {
 	v.SetDefault("server.host", "0.0.0.0")
 	v.SetDefault("server.port", 8340)
 	v.SetDefault("server.mode", "release")
+	v.SetDefault("server.external_url", "")
 	v.SetDefault("database.path", "./data/backupx.db")
 	v.SetDefault("security.jwt_expire", "24h")
 	v.SetDefault("backup.temp_dir", "/tmp/backupx")

server/internal/config/config_test.go

@@ -1,6 +1,10 @@
 package config
 
-import "testing"
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
 
 func TestLoadUsesDefaultsWithoutConfigFile(t *testing.T) {
 	cfg, err := Load("")
@@ -18,3 +22,33 @@ func TestLoadUsesDefaultsWithoutConfigFile(t *testing.T) {
 		t.Fatalf("expected default database path, got %s", cfg.Database.Path)
 	}
 }
+
+func TestLoadReadsServerExternalURLFromFile(t *testing.T) {
+	configPath := filepath.Join(t.TempDir(), "config.yaml")
+	content := []byte("server:\n  external_url: \"https://backup.example.com\"\n")
+	if err := os.WriteFile(configPath, content, 0o600); err != nil {
+		t.Fatalf("write config: %v", err)
+	}
+
+	cfg, err := Load(configPath)
+	if err != nil {
+		t.Fatalf("Load returned error: %v", err)
+	}
+
+	if cfg.Server.ExternalURL != "https://backup.example.com" {
+		t.Fatalf("expected external URL from config, got %q", cfg.Server.ExternalURL)
+	}
+}
+
+func TestLoadReadsServerExternalURLFromEnv(t *testing.T) {
+	t.Setenv("BACKUPX_SERVER_EXTERNAL_URL", "https://env-backup.example.com")
+
+	cfg, err := Load("")
+	if err != nil {
+		t.Fatalf("Load returned error: %v", err)
+	}
+
+	if cfg.Server.ExternalURL != "https://env-backup.example.com" {
+		t.Fatalf("expected external URL from env, got %q", cfg.Server.ExternalURL)
+	}
+}