Add management commands for semester deadline migration

- add update_semester_deadlines: Update quarterly deadlines to semester-based system
- add check_deadlines: Check current deadline status and validation
- includes dry-run mode and year filtering options
- provides clear progress feedback and summaries

Usage on VPS after GitHub Action deployment:
  python manage.py check_deadlines
  python manage.py update_semester_deadlines --dry-run
  python manage.py update_semester_deadlines

app/stiftung/management/commands/check_deadlines.py

@@ -0,0 +1,77 @@
+from django.core.management.base import BaseCommand
+from collections import defaultdict
+from stiftung.models import VierteljahresNachweis
+
+
+class Command(BaseCommand):
+    help = 'Check current quarterly confirmation deadline status'
+
+    def add_arguments(self, parser):
+        parser.add_argument(
+            '--year',
+            type=int,
+            help='Check deadlines for specific year (default: current and next year)',
+        )
+
+    def handle(self, *args, **options):
+        from datetime import date
+        
+        year_filter = options['year']
+        current_year = date.today().year
+        
+        if year_filter:
+            years_to_check = [year_filter]
+        else:
+            years_to_check = [current_year, current_year + 1]
+        
+        self.stdout.write('Current Quarterly Confirmation Deadline Status')
+        self.stdout.write('=' * 50)
+        
+        for year in years_to_check:
+            self.stdout.write(f'\nYear: {year}')
+            self.stdout.write('-' * 20)
+            
+            deadlines_by_quarter = defaultdict(set)
+            status_by_quarter = defaultdict(list)
+            
+            records = VierteljahresNachweis.objects.filter(jahr=year).order_by('quartal', 'destinataer__nachname')
+            
+            if not records:
+                self.stdout.write('  No records found for this year')
+                continue
+                
+            for record in records:
+                deadlines_by_quarter[record.quartal].add(record.faelligkeitsdatum)
+                status_by_quarter[record.quartal].append(record.status)
+            
+            # Show deadline summary
+            expected_deadlines = {
+                1: f'{year}-03-15',  # March 15
+                2: f'{year}-06-15',  # June 15
+                3: f'{year}-09-15',  # September 15
+                4: f'{year}-12-15',  # December 15
+            }
+            
+            for quarter in sorted(deadlines_by_quarter.keys()):
+                unique_deadlines = list(deadlines_by_quarter[quarter])
+                expected = expected_deadlines[quarter]
+                record_count = len([s for s in status_by_quarter[quarter]])
+                
+                if len(unique_deadlines) == 1 and str(unique_deadlines[0]) == expected:
+                    status_icon = self.style.SUCCESS('✓')
+                    status_text = 'CORRECT'
+                else:
+                    status_icon = self.style.ERROR('✗')
+                    status_text = 'NEEDS UPDATE'
+                
+                self.stdout.write(f'  Q{quarter}: {status_icon} {unique_deadlines[0]} (expected: {expected}) - {record_count} records - {status_text}')
+        
+        self.stdout.write('\nExpected semester-based deadline structure:')
+        self.stdout.write('  Q1: March 15   (Primary semester submission)')
+        self.stdout.write('  Q2: June 15    (Auto-approved when Q1 approved)')  
+        self.stdout.write('  Q3: September 15 (Primary semester submission)')
+        self.stdout.write('  Q4: December 15  (Auto-approved when Q3 approved)')
+        
+        self.stdout.write(f'\nTo update deadlines, run:')
+        self.stdout.write(f'  python manage.py update_semester_deadlines --dry-run  # Preview changes')
+        self.stdout.write(f'  python manage.py update_semester_deadlines           # Apply changes')