Implement advance payment schedule for quarterly payments

BREAKING CHANGE: Payment due dates now follow advance payment schedule:
- Q1 payments: Due December 15 of previous year
- Q2 payments: Due March 15 of same year
- Q3 payments: Due June 15 of same year
- Q4 payments: Due September 15 of same year

Changes:
- Updated create_quarterly_support_payment() to use advance schedule
- Enhanced diagnostic commands to check advance payment dates
- Added fix_advance_payment_dates command for comprehensive fixes
- Updated fix_q4_payment_dates to correct Q4 from 31.12 to 15.09

This aligns with semester-based document submissions while maintaining
the advance payment system where payments are made before each quarter.

app/stiftung/management/commands/diagnose_deadlines.py

@@ -89,14 +89,29 @@ class Command(BaseCommand):
         else:
             self.stdout.write('  ❌ Run: python manage.py update_semester_deadlines')
         
-        # Check if payments need updating
+        # Check if payments need updating (advance payment schedule)
-        wrong_payments = DestinataerUnterstuetzung.objects.filter(
+        wrong_q4_payments = DestinataerUnterstuetzung.objects.filter(
             faellig_am='2025-12-31',
             beschreibung__icontains='Q4/2025'
         ).count()
         
-        if wrong_payments > 0:
+        # Also check for other quarters with wrong advance payment dates
-            self.stdout.write(f'  ❌ {wrong_payments} payments have wrong due date (31.12 instead of 15.12)')
+        wrong_q1_payments = DestinataerUnterstuetzung.objects.filter(
+            faellig_am='2025-03-31',
+            beschreibung__icontains='Q1/2025'
+        ).count()
+        
+        if wrong_q4_payments > 0:
+            self.stdout.write(f'  ❌ {wrong_q4_payments} Q4 payments have wrong due date (31.12 instead of 15.09)')
             self.stdout.write('     Run: python manage.py fix_q4_payment_dates')
+        elif wrong_q1_payments > 0:
+            self.stdout.write(f'  ❌ {wrong_q1_payments} Q1 payments have wrong due date (advance payment schedule)')
         else:
             self.stdout.write('  ✓ Payment due dates appear correct')
+        
+        # Show the advance payment schedule
+        self.stdout.write('\nAdvance Payment Schedule:')
+        self.stdout.write('  Q1 payments: Due December 15 of previous year')
+        self.stdout.write('  Q2 payments: Due March 15 of same year')
+        self.stdout.write('  Q3 payments: Due June 15 of same year')
+        self.stdout.write('  Q4 payments: Due September 15 of same year')

app/stiftung/management/commands/fix_advance_payment_dates.py

@@ -0,0 +1,99 @@
+from django.core.management.base import BaseCommand
+from django.db import transaction
+from datetime import date
+from stiftung.models import DestinataerUnterstuetzung
+
+
+class Command(BaseCommand):
+    help = 'Fix all quarterly payment dates to use advance payment schedule'
+
+    def add_arguments(self, parser):
+        parser.add_argument(
+            '--dry-run',
+            action='store_true',
+            help='Show what would be updated without making changes',
+        )
+        parser.add_argument(
+            '--year',
+            type=int,
+            default=2025,
+            help='Year to fix payments for (default: 2025)',
+        )
+
+    def handle(self, *args, **options):
+        dry_run = options['dry_run']
+        year = options['year']
+        
+        if dry_run:
+            self.stdout.write(self.style.WARNING('DRY RUN MODE - No changes will be made'))
+        
+        self.stdout.write(f'Checking quarterly payments for {year} using advance payment schedule...')
+        
+        # Define correct advance payment schedule
+        advance_schedule = {
+            f'Q1/{year}': date(year - 1, 12, 15),  # Q1 paid in December of previous year
+            f'Q2/{year}': date(year, 3, 15),       # Q2 paid in March of same year  
+            f'Q3/{year}': date(year, 6, 15),       # Q3 paid in June of same year
+            f'Q4/{year}': date(year, 9, 15),       # Q4 paid in September of same year
+        }
+        
+        total_fixed = 0
+        
+        for quarter_desc, correct_date in advance_schedule.items():
+            # Find payments for this quarter that have wrong dates
+            wrong_payments = DestinataerUnterstuetzung.objects.filter(
+                beschreibung__icontains=quarter_desc
+            ).exclude(faellig_am=correct_date)
+            
+            quarter_count = wrong_payments.count()
+            
+            if quarter_count > 0:
+                self.stdout.write(f'\n{quarter_desc}: Found {quarter_count} payments with wrong due dates')
+                
+                with transaction.atomic():
+                    for payment in wrong_payments:
+                        old_date = payment.faellig_am
+                        
+                        if not dry_run:
+                            payment.faellig_am = correct_date
+                            payment.save(update_fields=['faellig_am'])
+                        
+                        total_fixed += 1
+                        
+                        # Show some examples
+                        if total_fixed <= 10 or options['verbosity'] >= 2:
+                            self.stdout.write(
+                                f'  {payment.destinataer.get_full_name()}: '
+                                f'{old_date} → {correct_date} ({payment.betrag}€)'
+                            )
+            else:
+                self.stdout.write(f'{quarter_desc}: ✓ All payments have correct due date ({correct_date})')
+        
+        # Summary
+        if total_fixed == 0:
+            self.stdout.write(self.style.SUCCESS(f'\n✓ All {year} quarterly payments already have correct advance payment dates!'))
+        elif dry_run:
+            self.stdout.write(
+                self.style.WARNING(f'\nDRY RUN: Would fix {total_fixed} payment due dates')
+            )
+        else:
+            self.stdout.write(
+                self.style.SUCCESS(f'\n✅ Successfully fixed {total_fixed} payment due dates')
+            )
+        
+        # Show the advance payment schedule
+        self.stdout.write(f'\nAdvance Payment Schedule for {year}:')
+        for quarter_desc, due_date in advance_schedule.items():
+            self.stdout.write(f'  {quarter_desc}: Due {due_date.strftime("%B %d, %Y")}')
+        
+        # Check for other years that might need fixing
+        if year == 2025:
+            other_year_payments = DestinataerUnterstuetzung.objects.filter(
+                beschreibung__regex=r'Q[1-4]/202[6-9]'
+            ).exclude(
+                faellig_am__in=[date(2024, 12, 15), date(2026, 3, 15), date(2026, 6, 15), date(2026, 9, 15)]
+            ).count()
+            
+            if other_year_payments > 0:
+                self.stdout.write(f'\n⚠️  Found {other_year_payments} payments for other years that may need fixing')
+                self.stdout.write('   Run this command with --year 2026, etc. to fix other years')

app/stiftung/management/commands/fix_q4_payment_dates.py

@@ -20,7 +20,7 @@ class Command(BaseCommand):
         if dry_run:
             self.stdout.write(self.style.WARNING('DRY RUN MODE - No changes will be made'))
         
-        # Find all Q4/2025 payments with wrong due date (Dec 31 instead of Dec 15)
+        # Find all Q4/2025 payments with wrong due date (Dec 31 instead of Sep 15)
         wrong_payments = DestinataerUnterstuetzung.objects.filter(
             faellig_am='2025-12-31',
             beschreibung__icontains='Q4/2025'
@@ -34,7 +34,7 @@ class Command(BaseCommand):
             return
         
         updated_count = 0
-        correct_date = date(2025, 12, 15)  # December 15, 2025
+        correct_date = date(2025, 9, 15)  # September 15, 2025 (advance payment for Q4)
         
         with transaction.atomic():
             for payment in wrong_payments:
@@ -57,13 +57,13 @@ class Command(BaseCommand):
         if dry_run:
             self.stdout.write(
                 self.style.SUCCESS(
-                    f'DRY RUN: Would update {updated_count} payment due dates from 31.12.2025 to 15.12.2025'
+                    f'DRY RUN: Would update {updated_count} payment due dates from 31.12.2025 to 15.09.2025'
                 )
             )
         else:
             self.stdout.write(
                 self.style.SUCCESS(
-                    f'Successfully updated {updated_count} payment due dates to December 15, 2025'
+                    f'Successfully updated {updated_count} payment due dates to September 15, 2025'
                 )
             )
         
@@ -86,8 +86,8 @@ class Command(BaseCommand):
                 self.stdout.write(f'  ⚠️  {quarter_desc}: {old_date_payments} payments still use old date {old_expected}')
                 self.stdout.write(f'      Should be: {new_expected}')
         
-        self.stdout.write('\nSemester-based payment schedule:')
+        self.stdout.write('\nAdvance payment schedule (payments made in advance):')
-        self.stdout.write('  Q1: March 15   (Spring semester)')
+        self.stdout.write('  Q1 payments: Due December 15 of previous year')
-        self.stdout.write('  Q2: June 15    (Auto-approved with Q1)')
+        self.stdout.write('  Q2 payments: Due March 15 of same year')
-        self.stdout.write('  Q3: September 15 (Fall semester)')  
+        self.stdout.write('  Q3 payments: Due June 15 of same year')  
-        self.stdout.write('  Q4: December 15  (Auto-approved with Q3)')
+        self.stdout.write('  Q4 payments: Due September 15 of same year')

app/stiftung/views.py

@@ -7484,6 +7484,7 @@ def create_quarterly_support_payment(nachweis):
     Get or create a single support payment for this quarterly confirmation
     Ensures only one payment exists per destinataer per quarter
     """
+    from datetime import date
     destinataer = nachweis.destinataer
     
     # Check if all requirements are met
@@ -7532,19 +7533,21 @@ def create_quarterly_support_payment(nachweis):
         if not default_konto:
             return None
     
-    # Calculate payment due date (last day of quarter)
+    # Calculate payment due date (advance payment schedule)
-    quarter_end_month = nachweis.quartal * 3
+    # Payments are made in advance according to semester schedule:
+    # September 15: Payment for Q4 of current year
+    # December 15: Payment for Q1 of next year  
+    # March 15: Payment for Q2 of current year
+    # June 15: Payment for Q3 of current year
     
-    if nachweis.quartal == 1:  # Q1: January-March (ends March 31)
+    if nachweis.quartal == 1:  # Q1 payment due December 15 of previous year
-        quarter_end_day = 31
+        payment_due_date = date(nachweis.jahr - 1, 12, 15)
-    elif nachweis.quartal == 2:  # Q2: April-June (ends June 30)
+    elif nachweis.quartal == 2:  # Q2 payment due March 15 of same year  
-        quarter_end_day = 30
+        payment_due_date = date(nachweis.jahr, 3, 15)
-    elif nachweis.quartal == 3:  # Q3: July-September (ends September 30)
+    elif nachweis.quartal == 3:  # Q3 payment due June 15 of same year
-        quarter_end_day = 30
+        payment_due_date = date(nachweis.jahr, 6, 15)
-    else:  # Q4: October-December (ends December 31)
+    else:  # Q4 payment due September 15 of same year
-        quarter_end_day = 31
+        payment_due_date = date(nachweis.jahr, 9, 15)
-    
-    payment_due_date = datetime(nachweis.jahr, quarter_end_month, quarter_end_day).date()
     
     # Create the support payment
     payment = DestinataerUnterstuetzung.objects.create(