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/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)
-    quarter_end_month = nachweis.quartal * 3
+    # Calculate payment due date (advance payment schedule)
+    # 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)
-        quarter_end_day = 31
-    elif nachweis.quartal == 2:  # Q2: April-June (ends June 30)
-        quarter_end_day = 30
-    elif nachweis.quartal == 3:  # Q3: July-September (ends September 30)
-        quarter_end_day = 30
-    else:  # Q4: October-December (ends December 31)
-        quarter_end_day = 31
-    
-    payment_due_date = datetime(nachweis.jahr, quarter_end_month, quarter_end_day).date()
+    if nachweis.quartal == 1:  # Q1 payment due December 15 of previous year
+        payment_due_date = date(nachweis.jahr - 1, 12, 15)
+    elif nachweis.quartal == 2:  # Q2 payment due March 15 of same year  
+        payment_due_date = date(nachweis.jahr, 3, 15)
+    elif nachweis.quartal == 3:  # Q3 payment due June 15 of same year
+        payment_due_date = date(nachweis.jahr, 6, 15)
+    else:  # Q4 payment due September 15 of same year
+        payment_due_date = date(nachweis.jahr, 9, 15)
     
     # Create the support payment
     payment = DestinataerUnterstuetzung.objects.create(