Fix semester system: 2x yearly docs, 4x yearly payments with Q4 on Sep 15

Document Submission (twice yearly):
- Q1 & Q2 documents: Due March 15 (Spring semester)
- Q3 & Q4 documents: Due September 15 (Fall semester)

Payment Schedule (quarterly):
- Q1 payments: Due March 15
- Q2 payments: Due June 15
- Q3 payments: Due September 15
- Q4 payments: Due September 15 (aligned with Fall semester docs)

This gives the desired Q4 payment due date of 15.09.2025 while maintaining
quarterly payment frequency and semester-based document submissions.

app/stiftung/management/commands/fix_advance_payment_dates.py

@@ -29,17 +29,17 @@ class Command(BaseCommand):
         
         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
+        # Define correct quarterly payment schedule
+        quarterly_schedule = {
+            f'Q1/{year}': date(year, 3, 15),       # Q1 payment due March 15
+            f'Q2/{year}': date(year, 6, 15),       # Q2 payment due June 15  
+            f'Q3/{year}': date(year, 9, 15),       # Q3 payment due September 15
+            f'Q4/{year}': date(year, 9, 15),       # Q4 payment due September 15 (semester alignment)
         }
         
         total_fixed = 0
         
-        for quarter_desc, correct_date in advance_schedule.items():
+        for quarter_desc, correct_date in quarterly_schedule.items():
             # Find payments for this quarter that have wrong dates
             wrong_payments = DestinataerUnterstuetzung.objects.filter(
                 beschreibung__icontains=quarter_desc
@@ -81,9 +81,9 @@ class Command(BaseCommand):
                 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():
+        # Show the quarterly payment schedule
+        self.stdout.write(f'\nQuarterly Payment Schedule for {year}:')
+        for quarter_desc, due_date in quarterly_schedule.items():
             self.stdout.write(f'  {quarter_desc}: Due {due_date.strftime("%B %d, %Y")}')
         
         # Check for other years that might need fixing
@@ -91,7 +91,7 @@ class Command(BaseCommand):
             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)]
+                faellig_am__in=[date(2026, 3, 15), date(2026, 6, 15), date(2026, 9, 15), date(2026, 9, 15)]
             ).count()
             
             if other_year_payments > 0: