feat: add controller self-update mechanism (v0.16.0)

New selfupdate package: version parsing, audit state file, updater with
Gitea registry V2 check, docker pull + compose rewrite + compose up flow.

- API: /api/selfupdate/{status,check,update} with session+bearer auth
- UI: Settings "Verzió és frissítés" card with check/install buttons + JS polling
- Scheduler: periodic check (6h default) + optional daily auto-update
- Notifications: success/failure on post-update startup verification
- Alert: info banner when update available
- docker-compose.yml: add directory bind mount for compose file access

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

controller/internal/web/alerts.go

@@ -40,7 +40,7 @@ func NewAlertManager(logger *log.Logger) *AlertManager {
 
 // Refresh regenerates alerts from the latest health check report and config state.
 // Called after each health check cycle (every 5 minutes).
-func (am *AlertManager) Refresh(report *monitor.HealthReport, cfg *config.Config, backupMgr *backup.Manager) {
+func (am *AlertManager) Refresh(report *monitor.HealthReport, cfg *config.Config, backupMgr *backup.Manager, updateAvailable bool, latestVersion string) {
 	var alerts []Alert
 
 	// From health check issues (critical)
@@ -97,6 +97,17 @@ func (am *AlertManager) Refresh(report *monitor.HealthReport, cfg *config.Config
 		})
 	}
 
+	// Update available
+	if updateAvailable && latestVersion != "" {
+		alerts = append(alerts, Alert{
+			ID:       "update-available",
+			Level:    "info",
+			Message:  fmt.Sprintf("Új controller verzió elérhető: %s", latestVersion),
+			Link:     "/settings",
+			LinkText: "Frissítés",
+		})
+	}
+
 	// Sort: errors first, then warnings, then info
 	sortAlerts(alerts)