Fix nachweis-board TemplateSyntaxError: add missing get_item filter

The nachweis_board.html template used a get_item filter that was never
defined. Added it to help_tags.py and loaded the tag library in the template.

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

app/templates/stiftung/nachweis_board.html

@@ -1,5 +1,5 @@
 {% extends 'base.html' %}
-{% load static %}
+{% load static help_tags %}
 
 {% block title %}Nachweis-Board – Stiftungsverwaltung{% endblock %}