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/reconcile_balances.py

@@ -0,0 +1,145 @@
+"""
+Management command to reconcile account balances with actual transactions.
+
+This command will:
+1. Calculate the correct balance based on all bank transactions
+2. Update the account balance to match the calculated balance
+3. Show discrepancies between stored and calculated balances
+
+Usage:
+    python manage.py reconcile_balances
+"""
+
+from django.core.management.base import BaseCommand
+from django.utils import timezone
+from decimal import Decimal
+
+from stiftung.models import StiftungsKonto, BankTransaction
+
+
+class Command(BaseCommand):
+    help = 'Reconcile account balances with bank transactions'
+
+    def add_arguments(self, parser):
+        parser.add_argument(
+            '--dry-run',
+            action='store_true',
+            help='Show what would be done without making changes',
+        )
+        parser.add_argument(
+            '--account',
+            type=str,
+            help='Only process specific account (by kontoname)',
+        )
+
+    def handle(self, *args, **options):
+        dry_run = options['dry_run']
+        account_filter = options.get('account')
+        
+        self.stdout.write(
+            self.style.SUCCESS('🔍 Reconciling account balances with transactions...')
+        )
+        
+        if dry_run:
+            self.stdout.write(
+                self.style.WARNING('DRY RUN MODE - No changes will be made')
+            )
+        
+        # Get accounts to process
+        accounts_query = StiftungsKonto.objects.filter(aktiv=True)
+        if account_filter:
+            accounts_query = accounts_query.filter(kontoname__icontains=account_filter)
+        
+        accounts = accounts_query.all()
+        
+        if not accounts:
+            self.stdout.write(
+                self.style.ERROR('No accounts found matching criteria')
+            )
+            return
+        
+        self.stdout.write(f"Processing {accounts.count()} account(s)...")
+        
+        total_discrepancies = 0
+        fixed_accounts = 0
+        
+        for account in accounts:
+            self.stdout.write(f"\n🏦 {account.bank_name} - {account.kontoname}")
+            self.stdout.write(f"   Stored Balance: €{account.saldo}")
+            self.stdout.write(f"   Last Updated: {account.saldo_datum}")
+            
+            # Calculate balance from transactions
+            transactions = BankTransaction.objects.filter(konto=account).order_by('datum')
+            calculated_balance = Decimal('0.00')
+            transaction_count = transactions.count()
+            
+            for transaction in transactions:
+                calculated_balance += transaction.betrag
+            
+            self.stdout.write(f"   Transactions: {transaction_count}")
+            self.stdout.write(f"   Calculated Balance: €{calculated_balance}")
+            
+            discrepancy = account.saldo - calculated_balance
+            if discrepancy != 0:
+                total_discrepancies += 1
+                self.stdout.write(
+                    self.style.WARNING(f"   ⚠️ Discrepancy: €{discrepancy}")
+                )
+                
+                if not dry_run:
+                    # Update the account balance
+                    old_balance = account.saldo
+                    account.saldo = calculated_balance
+                    
+                    # Update the balance date to the latest transaction date or today
+                    if transactions.exists():
+                        latest_transaction = transactions.order_by('-datum').first()
+                        account.saldo_datum = latest_transaction.datum
+                    else:
+                        account.saldo_datum = timezone.now().date()
+                    
+                    account.save()
+                    fixed_accounts += 1
+                    
+                    self.stdout.write(
+                        self.style.SUCCESS(
+                            f"   ✅ Updated: €{old_balance} → €{calculated_balance}"
+                        )
+                    )
+                else:
+                    self.stdout.write(
+                        f"   📝 Would update: €{account.saldo} → €{calculated_balance}"
+                    )
+            else:
+                self.stdout.write(
+                    self.style.SUCCESS("   ✅ Balance is correct")
+                )
+            
+            # Show recent transactions
+            if transaction_count > 0:
+                self.stdout.write("   Recent transactions:")
+                recent = transactions.order_by('-datum')[:3]
+                for trans in recent:
+                    self.stdout.write(
+                        f"     {trans.datum}: €{trans.betrag} - {trans.verwendungszweck[:50]}"
+                    )
+        
+        # Summary
+        self.stdout.write("\n" + "=" * 50)
+        if dry_run:
+            self.stdout.write(
+                f"📊 Found {total_discrepancies} account(s) with balance discrepancies"
+            )
+            if total_discrepancies > 0:
+                self.stdout.write("   Run without --dry-run to fix these discrepancies")
+        else:
+            if fixed_accounts > 0:
+                self.stdout.write(
+                    self.style.SUCCESS(
+                        f"🎉 Fixed {fixed_accounts} account balance(s)"
+                    )
+                )
+            else:
+                self.stdout.write(
+                    self.style.SUCCESS("✅ All account balances were already correct")
+                )