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.

test_sanity_check.py

@@ -0,0 +1,38 @@
+#!/usr/bin/env python
+"""
+Simple script to test Vorgaben sanity checking
+"""
+import os
+import sys
+import django
+
+# Setup Django
+os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'VorgabenUI.settings')
+django.setup()
+
+from dokumente.utils import check_vorgabe_conflicts, format_conflict_report
+
+
+def main():
+    print("Running Vorgaben sanity check...")
+    print("=" * 50)
+    
+    # Check for conflicts
+    conflicts = check_vorgabe_conflicts()
+    
+    # Generate and display report
+    report = format_conflict_report(conflicts, verbose=True)
+    print(report)
+    
+    print("=" * 50)
+    
+    if conflicts:
+        print(f"\n⚠️  Found {len(conflicts)} conflicts that need attention!")
+        sys.exit(1)
+    else:
+        print("✅ All Vorgaben are valid!")
+        sys.exit(0)
+
+
+if __name__ == "__main__":
+    main()