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/utils.py

@@ -0,0 +1,123 @@
+"""
+Utility functions for Vorgaben sanity checking
+"""
+import datetime
+from django.db.models import Count
+from itertools import combinations
+from dokumente.models import Vorgabe
+
+
+def check_vorgabe_conflicts():
+    """
+    Check for conflicts in Vorgaben.
+    
+    Main rule: If there are two Vorgaben with the same number, Thema and Dokument,
+    their valid_from and valid_to date ranges shouldn't intersect.
+    
+    Returns:
+        list: List of conflict dictionaries
+    """
+    conflicts = []
+    
+    # Find Vorgaben with same dokument, thema, and nummer
+    duplicate_groups = (
+        Vorgabe.objects.values('dokument', 'thema', 'nummer')
+        .annotate(count=Count('id'))
+        .filter(count__gt=1)
+    )
+    
+    for group in duplicate_groups:
+        # Get all Vorgaben in this group
+        vorgaben = Vorgabe.objects.filter(
+            dokument=group['dokument'],
+            thema=group['thema'], 
+            nummer=group['nummer']
+        )
+        
+        # Check all pairs for date range intersections
+        for vorgabe1, vorgabe2 in combinations(vorgaben, 2):
+            if date_ranges_intersect(
+                vorgabe1.gueltigkeit_von, vorgabe1.gueltigkeit_bis,
+                vorgabe2.gueltigkeit_von, vorgabe2.gueltigkeit_bis
+            ):
+                conflicts.append({
+                    'vorgabe1': vorgabe1,
+                    'vorgabe2': vorgabe2,
+                    'conflict_type': 'date_range_intersection',
+                    'message': f"Vorgaben {vorgabe1.Vorgabennummer()} and {vorgabe2.Vorgabennummer()} "
+                              f"have intersecting validity periods"
+                })
+    
+    return conflicts
+
+
+def date_ranges_intersect(start1, end1, start2, end2):
+    """
+    Check if two date ranges intersect.
+    None end date means open-ended range.
+    
+    Args:
+        start1, start2: Start dates
+        end1, end2: End dates (can be None for open-ended)
+        
+    Returns:
+        bool: True if ranges intersect
+    """
+    # If either start date is None, treat it as invalid case
+    if not start1 or not start2:
+        return False
+        
+    # If end date is None, treat it as far future
+    end1 = end1 or datetime.date.max
+    end2 = end2 or datetime.date.max
+    
+    # Ranges intersect if start1 <= end2 and start2 <= end1
+    return start1 <= end2 and start2 <= end1
+
+
+def format_conflict_report(conflicts, verbose=False):
+    """
+    Format conflicts into a readable report.
+    
+    Args:
+        conflicts: List of conflict dictionaries
+        verbose: Whether to show detailed information
+        
+    Returns:
+        str: Formatted report
+    """
+    if not conflicts:
+        return "✓ No conflicts found in Vorgaben"
+    
+    lines = [f"Found {len(conflicts)} conflicts:"]
+    
+    for i, conflict in enumerate(conflicts, 1):
+        lines.append(f"\n{i}. {conflict['message']}")
+        
+        if verbose:
+            v1 = conflict['vorgabe1']
+            v2 = conflict['vorgabe2']
+            
+            lines.append(f"   Vorgabe 1: {v1.Vorgabennummer()}")
+            lines.append(f"   Valid from: {v1.gueltigkeit_von} to {v1.gueltigkeit_bis or 'unlimited'}")
+            lines.append(f"   Title: {v1.titel}")
+            
+            lines.append(f"   Vorgabe 2: {v2.Vorgabennummer()}")
+            lines.append(f"   Valid from: {v2.gueltigkeit_von} to {v2.gueltigkeit_bis or 'unlimited'}")
+            lines.append(f"   Title: {v2.titel}")
+        
+        # Show the overlapping period
+        v1 = conflict['vorgabe1']
+        v2 = conflict['vorgabe2']
+        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:
+            lines.append(f"   Overlap: {overlap_start} to {overlap_end}")
+        else:
+            lines.append(f"   Overlap starts: {overlap_start} (no end)")
+    
+    return "\n".join(lines)