Fix quarterly payment schedule to correct advance payment dates

- Q1: Due December 15 (previous year)
- Q2: Due March 15
- Q3: Due June 15
- Q4: Due September 15

Added new management command fix_quarterly_payment_schedule to update existing payments

app/stiftung/management/commands/fix_quarterly_payment_schedule.py

@@ -0,0 +1,107 @@
+from django.core.management.base import BaseCommand
+from django.db.models import Q
+from datetime import date
+from stiftung.models import DestinataerUnterstuetzung
+
+
+class Command(BaseCommand):
+    help = 'Fix quarterly payment due dates to correct advance payment schedule'
+
+    def add_arguments(self, parser):
+        parser.add_argument(
+            '--dry-run',
+            action='store_true',
+            help='Show what would be changed without making actual changes',
+        )
+
+    def handle(self, *args, **options):
+        dry_run = options['dry_run']
+        
+        if dry_run:
+            self.stdout.write(
+                self.style.WARNING('DRY RUN MODE - No changes will be made')
+            )
+        
+        # Find all quarterly support payments that need fixing
+        payments_to_fix = DestinataerUnterstuetzung.objects.filter(
+            beschreibung__contains="Vierteljährliche Unterstützung"
+        ).exclude(
+            beschreibung__contains="Korrektur"  # Avoid fixing already corrected payments
+        )
+        
+        total_fixed = 0
+        
+        for payment in payments_to_fix:
+            # Extract quarter and year from description
+            desc = payment.beschreibung
+            try:
+                # Look for pattern like "Q1/2025", "Q2/2025", etc.
+                import re
+                match = re.search(r'Q(\d)/(\d{4})', desc)
+                if not match:
+                    continue
+                    
+                quartal = int(match.group(1))
+                jahr = int(match.group(2))
+                
+                # Calculate correct due date based on advance payment schedule
+                if quartal == 1:  # Q1 payment due December 15 of previous year
+                    correct_due_date = date(jahr - 1, 12, 15)
+                elif quartal == 2:  # Q2 payment due March 15
+                    correct_due_date = date(jahr, 3, 15)
+                elif quartal == 3:  # Q3 payment due June 15
+                    correct_due_date = date(jahr, 6, 15)
+                elif quartal == 4:  # Q4 payment due September 15
+                    correct_due_date = date(jahr, 9, 15)
+                else:
+                    continue
+                    
+                # Check if this payment needs updating
+                if payment.faellig_am != correct_due_date:
+                    old_date = payment.faellig_am
+                    
+                    if dry_run:
+                        self.stdout.write(
+                            f"Would update {payment.destinataer.get_full_name()} "
+                            f"Q{quartal}/{jahr} payment: {old_date} -> {correct_due_date}"
+                        )
+                    else:
+                        payment.faellig_am = correct_due_date
+                        payment.save()
+                        
+                        self.stdout.write(
+                            self.style.SUCCESS(
+                                f"Updated {payment.destinataer.get_full_name()} "
+                                f"Q{quartal}/{jahr} payment: {old_date} -> {correct_due_date}"
+                            )
+                        )
+                    
+                    total_fixed += 1
+                    
+            except (ValueError, AttributeError) as e:
+                self.stdout.write(
+                    self.style.WARNING(
+                        f"Could not parse quarter/year from: {desc} - {e}"
+                    )
+                )
+                continue
+        
+        if dry_run:
+            self.stdout.write(
+                self.style.WARNING(
+                    f"Would fix {total_fixed} payment due dates"
+                )
+            )
+        else:
+            self.stdout.write(
+                self.style.SUCCESS(
+                    f"Successfully fixed {total_fixed} payment due dates"
+                )
+            )
+            
+        # Show the expected schedule
+        self.stdout.write("\nCorrect advance payment schedule:")
+        self.stdout.write("Q1: Due December 15 (previous year)")
+        self.stdout.write("Q2: Due March 15")  
+        self.stdout.write("Q3: Due June 15")
+        self.stdout.write("Q4: Due September 15")