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/__init__.py

@@ -0,0 +1,3 @@
+"""
+System app package
+"""

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',)
+        }

backend/igny8_core/business/system/apps.py

@@ -0,0 +1,11 @@
+"""
+System app configuration
+"""
+from django.apps import AppConfig
+
+
+class SystemConfig(AppConfig):
+    default_auto_field = 'django.db.models.BigAutoField'
+    name = 'igny8_core.business.system'
+    label = 'debug_system'
+    verbose_name = 'Debug & System Settings'

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',
+            },
+        ),
+    ]

backend/igny8_core/business/system/models.py

@@ -0,0 +1,86 @@
+"""
+System-wide settings and configuration models
+"""
+from django.db import models
+from django.conf import settings
+from django.core.cache import cache
+
+
+class DebugConfiguration(models.Model):
+    """
+    System-wide debug configuration (Singleton).
+    Controls verbose logging and debugging features.
+    """
+    # Debug settings
+    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)"
+    )
+    
+    # Audit fields
+    updated_at = models.DateTimeField(auto_now=True)
+    updated_by = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.SET_NULL,
+        null=True,
+        blank=True,
+        help_text="Admin who last updated"
+    )
+    
+    class Meta:
+        app_label = 'debug_system'
+        db_table = 'igny8_debug_configuration'
+        verbose_name = 'Debug Configuration'
+        verbose_name_plural = 'Debug Configuration'
+    
+    def save(self, *args, **kwargs):
+        """Enforce singleton pattern and clear cache on save"""
+        self.pk = 1
+        super().save(*args, **kwargs)
+        # Clear ALL debug-related caches when settings change
+        cache.delete('debug_config')
+        cache.delete('debug_enabled')
+        cache.delete('debug_first_worker_pid')  # Reset worker selection
+    
+    @classmethod
+    def get_config(cls):
+        """Get or create the singleton config (cached)"""
+        config = cache.get('debug_config')
+        if config is None:
+            config, created = cls.objects.get_or_create(pk=1)
+            cache.set('debug_config', config, 300)  # Cache for 5 minutes
+        return config
+    
+    @classmethod
+    def is_debug_enabled(cls):
+        """Fast check if debug logging is enabled (cached for performance)"""
+        enabled = cache.get('debug_enabled')
+        if enabled is None:
+            config = cls.get_config()
+            enabled = config.enable_debug_logging
+            cache.set('debug_enabled', enabled, 60)  # Cache for 1 minute
+        return enabled
+    
+    def __str__(self):
+        status = "ENABLED" if self.enable_debug_logging else "DISABLED"
+        return f"Debug Configuration ({status})"