Hub: add POST /api/v1/notify endpoint for customer notifications

- New notification relay endpoint: receives events from customer controllers,
  looks up customer email preferences, sends via Resend HTTP API
- New tables: customer_notifications (per-customer email + event prefs),
  notification_log (audit trail for all notification attempts)
- Hungarian email template with severity, event type, timestamp
- Config: notifications.resend_api_key + notifications.from_email
- Test events always pass event-type filter

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

hub/cmd/hub/main.go

@@ -31,6 +31,10 @@ type Config struct {
 	API struct {
 		ReportAPIKey string `yaml:"report_api_key"`
 	} `yaml:"api"`
+	Notifications struct {
+		ResendAPIKey string `yaml:"resend_api_key"`
+		FromEmail    string `yaml:"from_email"`
+	} `yaml:"notifications"`
 	Retention struct {
 		MaxDays       int    `yaml:"max_days"`
 		PruneSchedule string `yaml:"prune_schedule"`
@@ -79,7 +83,7 @@ func main() {
 	}
 
 	// Initialize handlers
-	apiHandler := api.New(dataStore, cfg.API.ReportAPIKey, logger)
+	apiHandler := api.New(dataStore, cfg.API.ReportAPIKey, cfg.Notifications.ResendAPIKey, cfg.Notifications.FromEmail, logger)
 	webServer := web.New(dataStore, cfg.Auth.PasswordHash, staleThreshold, logger)
 
 	// Build HTTP mux
@@ -177,6 +181,9 @@ func loadConfig(path string, logger *log.Logger) *Config {
 	if cfg.Alerting.StaleThreshold == "" {
 		cfg.Alerting.StaleThreshold = "30m"
 	}
+	if cfg.Notifications.FromEmail == "" {
+		cfg.Notifications.FromEmail = "monitoring@felhom.eu"
+	}
 
 	return cfg
 }