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/configs/hub.yaml.example

@@ -17,6 +17,11 @@ api:
   # Bearer token required for report ingest (POST /api/v1/report)
   report_api_key: ""
 
+# --- Notifications ---
+notifications:
+  resend_api_key: ""           # Resend.com API key for sending notification emails
+  from_email: "monitoring@felhom.eu"  # Sender address for notification emails
+
 # --- Data retention ---
 retention:
   max_days: 90               # Keep 90 days of report history