feat(config): add MODE env var to select compose or swarm operation

internal/config/config.go

@@ -16,6 +16,7 @@ type Config struct {
 	LogLevel      string
 	UpdateTimeout time.Duration
 	OptInLabel    string
+	Mode          string
 }
 
 // Load reads configuration from environment variables, applies defaults,
@@ -28,12 +29,17 @@ func Load() (*Config, error) {
 		Port:       getenvDefault("PORT", "8080"),
 		LogLevel:   getenvDefault("LOG_LEVEL", "info"),
 		OptInLabel: getenvDefault("OPT_IN_LABEL", "se.shcizo.auto-update"),
+		Mode:       getenvDefault("MODE", "compose"),
 	}
 
 	if cfg.APIKey == "" {
 		return nil, errors.New("UPDATER_API_KEY is required")
 	}
 
+	if cfg.Mode != "compose" && cfg.Mode != "swarm" {
+		return nil, fmt.Errorf("MODE %q is invalid: must be %q or %q", cfg.Mode, "compose", "swarm")
+	}
+
 	timeoutStr := getenvDefault("UPDATE_TIMEOUT", "5m")
 	d, err := time.ParseDuration(timeoutStr)
 	if err != nil {

internal/config/config_test.go

@@ -57,3 +57,26 @@ func TestLoad_InvalidTimeoutErrors(t *testing.T) {
 	require.Error(t, err)
 	require.Contains(t, err.Error(), "UPDATE_TIMEOUT")
 }
+
+func TestLoad_DefaultsToComposeMode(t *testing.T) {
+	t.Setenv("UPDATER_API_KEY", "secret")
+	cfg, err := config.Load()
+	require.NoError(t, err)
+	require.Equal(t, "compose", cfg.Mode)
+}
+
+func TestLoad_AcceptsSwarmMode(t *testing.T) {
+	t.Setenv("UPDATER_API_KEY", "secret")
+	t.Setenv("MODE", "swarm")
+	cfg, err := config.Load()
+	require.NoError(t, err)
+	require.Equal(t, "swarm", cfg.Mode)
+}
+
+func TestLoad_RejectsInvalidMode(t *testing.T) {
+	t.Setenv("UPDATER_API_KEY", "secret")
+	t.Setenv("MODE", "kubernetes")
+	_, err := config.Load()
+	require.Error(t, err)
+	require.Contains(t, err.Error(), "MODE")
+}