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/management/commands/create_sample_events.py

@@ -0,0 +1,58 @@
+from django.core.management.base import BaseCommand
+from stiftung.models import StiftungsKalenderEintrag
+from datetime import date, timedelta
+
+
+class Command(BaseCommand):
+    help = 'Creates sample calendar events for testing'
+
+    def handle(self, *args, **options):
+        today = date.today()
+        
+        events = [
+            {
+                'titel': 'Vorstandssitzung',
+                'beschreibung': 'Monatliche Vorstandssitzung zur Besprechung aktueller Stiftungsangelegenheiten',
+                'datum': today + timedelta(days=7),
+                'kategorie': 'termin',
+                'prioritaet': 'hoch',
+            },
+            {
+                'titel': 'Zahlungserinnerung Familie Müller',
+                'beschreibung': 'Quartalsweise Unterstützung €500',
+                'datum': today + timedelta(days=3),
+                'kategorie': 'zahlung',
+                'prioritaet': 'kritisch',
+            },
+            {
+                'titel': 'Pachtvertrag Müller läuft aus',
+                'beschreibung': 'Vertrag für Grundstück A123 muss verlängert werden',
+                'datum': today + timedelta(days=30),
+                'kategorie': 'vertrag',
+                'prioritaet': 'hoch',
+            },
+            {
+                'titel': 'Geburtstag Maria Schmidt',
+                'beschreibung': '75. Geburtstag',
+                'datum': today + timedelta(days=5),
+                'kategorie': 'geburtstag',
+                'prioritaet': 'normal',
+            }
+        ]
+        
+        created_count = 0
+        for event_data in events:
+            event, created = StiftungsKalenderEintrag.objects.get_or_create(
+                titel=event_data['titel'],
+                defaults=event_data
+            )
+            if created:
+                created_count += 1
+                self.stdout.write(f'Created: {event.titel}')
+            else:
+                self.stdout.write(f'Already exists: {event.titel}')
+        
+        total = StiftungsKalenderEintrag.objects.count()
+        self.stdout.write(
+            self.style.SUCCESS(f'✅ Created {created_count} new events. Total: {total}')
+        )