Fix payment system balance integration and add calendar functionality

- Implement automated payment tracking with Django signals
- Fix duplicate transaction creation with unique referenz system
- Add calendar system with CRUD operations and event management
- Reorganize navigation menu (rename sections, move admin functions)
- Replace Geschichte editor with EasyMDE markdown editor
- Add management commands for balance reconciliation
- Create missing transactions for previously paid payments
- Ensure account balances accurately reflect all payment activity

Features added:
- Calendar entries creation and administration via menu
- Payment status tracking with automatic balance updates
- Duplicate prevention for payment transactions
- Markdown editor with live preview for Geschichte pages
- Database reconciliation tools for payment/balance sync

Bug fixes:
- Resolved IntegrityError on payment status changes
- Fixed missing account balance updates for paid payments
- Prevented duplicate balance deductions on re-saves
- Corrected menu structure and admin function placement

app/stiftung/forms.py

@@ -1648,14 +1648,29 @@ class GeschichteSeiteForm(forms.ModelForm):
     
     def clean_slug(self):
         slug = self.cleaned_data.get('slug')
-        titel = self.cleaned_data.get('titel')
+        titel = self.cleaned_data.get('titel', '')
         
         if not slug and titel:
             # Auto-generate slug from title
             from django.utils.text import slugify
             slug = slugify(titel)
         
+        if not slug:
+            raise forms.ValidationError('Slug ist erforderlich. Bitte geben Sie einen Titel ein.')
+            
         return slug
+    
+    def clean(self):
+        cleaned_data = super().clean()
+        titel = cleaned_data.get('titel', '')
+        slug = cleaned_data.get('slug', '')
+        
+        # Auto-generate slug if empty
+        if titel and not slug:
+            from django.utils.text import slugify
+            cleaned_data['slug'] = slugify(titel)
+            
+        return cleaned_data
 
 
 class GeschichteBildForm(forms.ModelForm):