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/migrations/0001_initial.py

@@ -0,0 +1,35 @@
+# Generated by Django 5.2.9 on 2025-12-23 02:32
+
+import django.db.models.deletion
+from django.conf import settings
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    initial = True
+
+    dependencies = [
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='DebugConfiguration',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('enable_debug_logging', models.BooleanField(default=False, help_text='Enable verbose debug logging to console (AI steps, detailed execution)')),
+                ('log_ai_steps', models.BooleanField(default=True, help_text='Log AI function execution steps (only when debug logging enabled)')),
+                ('log_api_requests', models.BooleanField(default=False, help_text='Log all API requests and responses (only when debug logging enabled)')),
+                ('log_database_queries', models.BooleanField(default=False, help_text='Log database queries (only when debug logging enabled)')),
+                ('log_celery_tasks', models.BooleanField(default=True, help_text='Log Celery task execution (only when debug logging enabled)')),
+                ('updated_at', models.DateTimeField(auto_now=True)),
+                ('updated_by', models.ForeignKey(blank=True, help_text='Admin who last updated', null=True, on_delete=django.db.models.deletion.SET_NULL, to=settings.AUTH_USER_MODEL)),
+            ],
+            options={
+                'verbose_name': 'Debug Configuration',
+                'verbose_name_plural': 'Debug Configuration',
+                'db_table': 'igny8_debug_configuration',
+            },
+        ),
+    ]