Phase 2: monitoring warnings, dashboard alerts & notification system

- Monitoring page: "Távoli monitoring" section showing healthcheck ping UUID
  configuration status (configured/not configured) for each of the 5 pings
- Alert manager: persistent dashboard banners on all pages generated from
  health check results, missing pings, and backup status
- Notification system: controller-side notifier sends events to hub relay,
  with cooldown tracking and event-type filtering
- Notification preferences UI: email, event checkboxes, cooldown settings
  on the settings page with test email functionality
- Settings refactored: shared settingsData() helper, NotificationPrefs
  struct with getter/setter and defaults

New files:
- controller/internal/web/alerts.go (AlertManager)
- controller/internal/notify/notifier.go (hub notification client)

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

controller/internal/settings/settings.go

@@ -26,8 +26,19 @@ type Settings struct {
 	DBValidations map[string]DBValidationCache `json:"db_validations,omitempty"`
 }
 
-// NotificationPrefs is a placeholder for Phase 2 notification settings.
-type NotificationPrefs struct{}
+// NotificationPrefs holds customer notification preferences.
+type NotificationPrefs struct {
+	Email         string   `json:"email,omitempty"`
+	EnabledEvents []string `json:"enabled_events,omitempty"`
+	CooldownHours int      `json:"cooldown_hours,omitempty"` // default: 6
+}
+
+// DefaultEnabledEvents are the events enabled by default for new customers.
+var DefaultEnabledEvents = []string{
+	"disk_warning",
+	"backup_failed",
+	"update_available",
+}
 
 // DBValidationCache holds cached DB dump validation results.
 type DBValidationCache struct {
@@ -127,3 +138,35 @@ func (s *Settings) SetDBValidation(filename string, cache DBValidationCache) err
 	s.DBValidations[filename] = cache
 	return s.save()
 }
+
+// GetNotificationPrefs returns a copy of the notification preferences.
+func (s *Settings) GetNotificationPrefs() *NotificationPrefs {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	if s.Notifications == nil {
+		return &NotificationPrefs{
+			EnabledEvents: DefaultEnabledEvents,
+			CooldownHours: 6,
+		}
+	}
+	prefs := *s.Notifications
+	if prefs.CooldownHours == 0 {
+		prefs.CooldownHours = 6
+	}
+	if prefs.EnabledEvents == nil {
+		prefs.EnabledEvents = DefaultEnabledEvents
+	}
+	// Return a copy of the slice
+	events := make([]string, len(prefs.EnabledEvents))
+	copy(events, prefs.EnabledEvents)
+	prefs.EnabledEvents = events
+	return &prefs
+}
+
+// SetNotificationPrefs updates notification preferences and saves to disk.
+func (s *Settings) SetNotificationPrefs(prefs *NotificationPrefs) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.Notifications = prefs
+	return s.save()
+}