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/stiftung/templatetags/help_tags.py

@@ -34,6 +34,17 @@ def help_box_exists(page_key):
     return HelpBox.get_help_for_page(page_key) is not None
 
 
+@register.filter
+def get_item(dictionary, key):
+    """Lookup a key in a dictionary, trying int conversion for numeric keys."""
+    if dictionary is None:
+        return None
+    result = dictionary.get(key)
+    if result is None and isinstance(key, str) and key.isdigit():
+        result = dictionary.get(int(key))
+    return result
+
+
 @register.filter
 def markdown_to_html(text):
     """Konvertiere Markdown-Text zu HTML"""