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/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)
+	}
+}