Add production diagnostic and fix commands for semester deadlines

- diagnose_deadlines: Check migration status, deadline correctness, and payment patterns
- fix_q4_payment_dates: Fix payments with wrong due dates (31.12 → 15.12)
- includes dry-run modes and detailed reporting
- helps identify why production dates haven't updated

Usage on production:
  python manage.py diagnose_deadlines
  python manage.py fix_q4_payment_dates --dry-run
  python manage.py fix_q4_payment_dates

app/stiftung/management/commands/fix_q4_payment_dates.py

@@ -0,0 +1,93 @@
+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 Q4 payment due dates from December 31 to December 15'
+
+    def add_arguments(self, parser):
+        parser.add_argument(
+            '--dry-run',
+            action='store_true',
+            help='Show what would be updated without making 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 Q4/2025 payments with wrong due date (Dec 31 instead of Dec 15)
+        wrong_payments = DestinataerUnterstuetzung.objects.filter(
+            faellig_am='2025-12-31',
+            beschreibung__icontains='Q4/2025'
+        )
+        
+        total_count = wrong_payments.count()
+        self.stdout.write(f'Found {total_count} Q4/2025 payments with wrong due date (31.12 instead of 15.12)')
+        
+        if total_count == 0:
+            self.stdout.write(self.style.SUCCESS('No payments need fixing!'))
+            return
+        
+        updated_count = 0
+        correct_date = date(2025, 12, 15)  # December 15, 2025
+        
+        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'])
+                
+                updated_count += 1
+                
+                # Show progress for every 5 updates or if verbose
+                if updated_count % 5 == 0 or options['verbosity'] >= 2:
+                    self.stdout.write(
+                        f'  {payment.destinataer.get_full_name()}: '
+                        f'{old_date} → {correct_date} ({payment.betrag}€)'
+                    )
+        
+        # Summary
+        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'
+                )
+            )
+        else:
+            self.stdout.write(
+                self.style.SUCCESS(
+                    f'Successfully updated {updated_count} payment due dates to December 15, 2025'
+                )
+            )
+        
+        # Also check for other quarters that might have wrong dates
+        self.stdout.write('\nChecking other quarters for potential issues:')
+        
+        quarter_checks = {
+            'Q1/2025': ('2025-02-15', '2025-03-15'),
+            'Q2/2025': ('2025-05-15', '2025-06-15'), 
+            'Q3/2025': ('2025-08-15', '2025-09-15'),
+        }
+        
+        for quarter_desc, (old_expected, new_expected) in quarter_checks.items():
+            old_date_payments = DestinataerUnterstuetzung.objects.filter(
+                faellig_am=old_expected,
+                beschreibung__icontains=quarter_desc
+            ).count()
+            
+            if old_date_payments > 0:
+                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('  Q1: March 15   (Spring semester)')
+        self.stdout.write('  Q2: June 15    (Auto-approved with Q1)')
+        self.stdout.write('  Q3: September 15 (Fall semester)')  
+        self.stdout.write('  Q4: December 15  (Auto-approved with Q3)')