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/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})"