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/modules/billing/migrations/0018_update_operation_choices.py

@@ -0,0 +1,28 @@
+# Generated by Django 5.2.9 on 2025-12-23 04:42
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('billing', '0017_add_history_tracking'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='creditcostconfig',
+            name='operation_type',
+            field=models.CharField(choices=[('clustering', 'Keyword Clustering'), ('idea_generation', 'Content Ideas Generation'), ('content_generation', 'Content Generation'), ('image_generation', 'Image Generation'), ('image_prompt_extraction', 'Image Prompt Extraction'), ('linking', 'Internal Linking'), ('optimization', 'Content Optimization'), ('reparse', 'Content Reparse'), ('site_structure_generation', 'Site Structure Generation'), ('site_page_generation', 'Site Page Generation')], help_text='AI operation type', max_length=50, unique=True),
+        ),
+        migrations.AlterField(
+            model_name='creditusagelog',
+            name='operation_type',
+            field=models.CharField(choices=[('clustering', 'Keyword Clustering'), ('idea_generation', 'Content Ideas Generation'), ('content_generation', 'Content Generation'), ('image_generation', 'Image Generation'), ('image_prompt_extraction', 'Image Prompt Extraction'), ('linking', 'Internal Linking'), ('optimization', 'Content Optimization'), ('reparse', 'Content Reparse'), ('site_structure_generation', 'Site Structure Generation'), ('site_page_generation', 'Site Page Generation'), ('ideas', 'Content Ideas Generation (Legacy)'), ('content', 'Content Generation (Legacy)'), ('images', 'Image Generation (Legacy)')], db_index=True, max_length=50),
+        ),
+        migrations.AlterField(
+            model_name='historicalcreditcostconfig',
+            name='operation_type',
+            field=models.CharField(choices=[('clustering', 'Keyword Clustering'), ('idea_generation', 'Content Ideas Generation'), ('content_generation', 'Content Generation'), ('image_generation', 'Image Generation'), ('image_prompt_extraction', 'Image Prompt Extraction'), ('linking', 'Internal Linking'), ('optimization', 'Content Optimization'), ('reparse', 'Content Reparse'), ('site_structure_generation', 'Site Structure Generation'), ('site_page_generation', 'Site Page Generation')], db_index=True, help_text='AI operation type', max_length=50),
+        ),
+    ]