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
-        wrong_payments = DestinataerUnterstuetzung.objects.filter(
+        # Check if payments need updating (advance payment schedule)
+        wrong_q4_payments = DestinataerUnterstuetzung.objects.filter(
             faellig_am='2025-12-31',
             beschreibung__icontains='Q4/2025'
         ).count()
         
-        if wrong_payments > 0:
-            self.stdout.write(f'  ❌ {wrong_payments} payments have wrong due date (31.12 instead of 15.12)')
+        # Also check for other quarters with wrong advance payment dates
+        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')
+            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')