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/management/commands/test_debug.py

@@ -0,0 +1,41 @@
+"""
+Management command to test debug logging system
+"""
+from django.core.management.base import BaseCommand
+from igny8_core.utils.debug import is_debug_enabled, debug_log, debug_log_ai_step
+
+
+class Command(BaseCommand):
+    help = 'Test debug logging system'
+
+    def handle(self, *args, **options):
+        self.stdout.write("=== Testing Debug System ===\n")
+        
+        # Check if debug is enabled
+        enabled = is_debug_enabled()
+        self.stdout.write(f"Debug enabled: {enabled}\n")
+        
+        if enabled:
+            self.stdout.write("Debug is ENABLED - logs should appear below:\n")
+            
+            # Test general debug log
+            debug_log("Test message 1 - General category", category='general')
+            
+            # Test AI step log
+            debug_log_ai_step("TEST_STEP", "Test AI step message", test_param="value123", count=42)
+            
+            # Test different categories
+            debug_log("Test message 2 - AI steps", category='ai_steps')
+            debug_log("Test message 3 - API requests", category='api_requests')
+            
+            self.stdout.write("\n✓ Test logs sent (check console output above)\n")
+        else:
+            self.stdout.write("Debug is DISABLED - no logs should appear\n")
+            
+            # Try to log anyway (should be skipped)
+            debug_log("This should NOT appear", category='general')
+            debug_log_ai_step("SKIP", "This should also NOT appear")
+            
+            self.stdout.write("✓ Logs were correctly skipped\n")
+        
+        self.stdout.write("\n=== Test Complete ===\n")