Add Vorgaben sanity check functionality

Implement comprehensive validation system to detect conflicting Vorgaben with overlapping validity periods.

Features:
- Static method Vorgabe.sanity_check_vorgaben() for global conflict detection
- Instance method Vorgabe.find_conflicts() for individual conflict checking
- Model validation via Vorgabe.clean() to prevent conflicting data
- Utility functions for date range intersection and conflict reporting
- Django management command 'sanity_check_vorgaben' for manual checks
- Comprehensive test suite with 17 new tests covering all functionality

Validation logic ensures Vorgaben with same dokument, thema, and nummer cannot have overlapping gueltigkeit_von/gueltigkeit_bis date ranges. Handles open-ended ranges (None end dates) and provides clear error messages.

Files added/modified:
- dokumente/models.py: Added sanity check methods and validation
- dokumente/utils.py: New utility functions for conflict detection
- dokumente/management/commands/sanity_check_vorgaben.py: New management command
- dokumente/tests.py: Added comprehensive test coverage
- test_sanity_check.py: Standalone test script

All tests pass (56/56) with no regressions.

dokumente/management/commands/sanity_check_vorgaben.py

@@ -0,0 +1,70 @@
+from django.core.management.base import BaseCommand
+from django.db import transaction
+from dokumente.models import Vorgabe
+import datetime
+
+
+class Command(BaseCommand):
+    help = 'Run sanity checks on Vorgaben to detect conflicts'
+
+    def add_arguments(self, parser):
+        parser.add_argument(
+            '--fix',
+            action='store_true',
+            help='Attempt to fix conflicts (not implemented yet)',
+        )
+        parser.add_argument(
+            '--verbose',
+            action='store_true',
+            help='Show detailed output',
+        )
+
+    def handle(self, *args, **options):
+        self.verbose = options['verbose']
+        
+        self.stdout.write(self.style.SUCCESS('Starting Vorgaben sanity check...'))
+        
+        # Run the sanity check
+        conflicts = Vorgabe.sanity_check_vorgaben()
+        
+        if not conflicts:
+            self.stdout.write(self.style.SUCCESS('✓ No conflicts found in Vorgaben'))
+            return
+        
+        self.stdout.write(
+            self.style.WARNING(f'Found {len(conflicts)} conflicts:')
+        )
+        
+        for i, conflict in enumerate(conflicts, 1):
+            self._display_conflict(i, conflict)
+        
+        if options['fix']:
+            self.stdout.write(self.style.ERROR('Auto-fix not implemented yet'))
+
+    def _display_conflict(self, index, conflict):
+        """Display a single conflict"""
+        v1 = conflict['vorgabe1']
+        v2 = conflict['vorgabe2']
+        
+        self.stdout.write(f"\n{index}. {conflict['message']}")
+        
+        if self.verbose:
+            self.stdout.write(f"   Vorgabe 1: {v1.Vorgabennummer()}")
+            self.stdout.write(f"   Valid from: {v1.gueltigkeit_von} to {v1.gueltigkeit_bis or 'unlimited'}")
+            self.stdout.write(f"   Title: {v1.titel}")
+            
+            self.stdout.write(f"   Vorgabe 2: {v2.Vorgabennummer()}")
+            self.stdout.write(f"   Valid from: {v2.gueltigkeit_von} to {v2.gueltigkeit_bis or 'unlimited'}")
+            self.stdout.write(f"   Title: {v2.titel}")
+        
+        # Show the overlapping period
+        overlap_start = max(v1.gueltigkeit_von, v2.gueltigkeit_von)
+        overlap_end = min(
+            v1.gueltigkeit_bis or datetime.date.max,
+            v2.gueltigkeit_bis or datetime.date.max
+        )
+        
+        if overlap_end != datetime.date.max:
+            self.stdout.write(f"   Overlap: {overlap_start} to {overlap_end}")
+        else:
+            self.stdout.write(f"   Overlap starts: {overlap_start} (no end)")