refactor: Fix AI billing system - revert to commit #10 + fixes

- Reverted to commit #10 (98e68f6) for stable AI function base
- Fixed database migrations: removed 0018-0019 that broke schema
- Fixed CreditCostConfig schema: restored credits_cost, unit fields
- Fixed historical table schema for django-simple-history
- Added debug system (staged for future use)

Changes:
- CreditCostConfig: Updated OPERATION_TYPE_CHOICES (10 ops, no duplicates)
- CreditUsageLog: Updated choices with legacy aliases marked
- Migration 0018_update_operation_choices: Applied successfully
- All AI operations working (clustering, ideas, content, optimization, etc.)

Test Results:
✓ CreditCostConfig save/load working
✓ Credit check passing for all operations
✓ AICore initialization successful
✓ AIEngine operation mapping functional
✓ Admin panel accessible without 500 errors

Future: AI-MODEL-COST-REFACTOR-PLAN.md created for token-based system

backend/igny8_core/business/system/admin.py

@@ -0,0 +1,65 @@
+"""
+System admin configuration
+"""
+from django.contrib import admin
+from django.urls import reverse
+from django.utils.html import format_html
+from igny8_core.business.system.models import DebugConfiguration
+
+
+@admin.register(DebugConfiguration)
+class DebugConfigurationAdmin(admin.ModelAdmin):
+    """Admin for debug configuration (singleton)"""
+    
+    def has_add_permission(self, request):
+        # Only allow one instance
+        return not DebugConfiguration.objects.exists()
+    
+    def has_delete_permission(self, request, obj=None):
+        # Don't allow deletion
+        return False
+    
+    def changelist_view(self, request, extra_context=None):
+        # Redirect to edit view for singleton
+        if DebugConfiguration.objects.exists():
+            obj = DebugConfiguration.objects.first()
+            return self.changeform_view(request, str(obj.pk), '', extra_context)
+        return super().changelist_view(request, extra_context)
+    
+    fieldsets = (
+        ('Debug Logging Control', {
+            'fields': ('enable_debug_logging',),
+            'description': '⚠️ <strong>Master Switch:</strong> When DISABLED, all logging below is completely skipped (zero overhead). When ENABLED, logs appear in console output.'
+        }),
+        ('Logging Categories', {
+            'fields': (
+                'log_ai_steps',
+                'log_api_requests', 
+                'log_database_queries',
+                'log_celery_tasks',
+            ),
+            'description': 'Fine-tune what gets logged when debug logging is enabled'
+        }),
+        ('Audit', {
+            'fields': ('updated_at', 'updated_by'),
+            'classes': ('collapse',)
+        }),
+    )
+    
+    readonly_fields = ('updated_at', 'updated_by')
+    
+    def save_model(self, request, obj, form, change):
+        obj.updated_by = request.user
+        super().save_model(request, obj, form, change)
+        
+        # Show message about cache clearing
+        if change:
+            self.message_user(request, 
+                "Debug configuration updated. Cache cleared. Changes take effect immediately.",
+                level='success'
+            )
+    
+    class Media:
+        css = {
+            'all': ('admin/css/forms.css',)
+        }