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/utils/debug.py

@@ -0,0 +1,128 @@
+"""
+Debug logging utilities
+Fast checks with minimal overhead when debug is disabled.
+"""
+from django.core.cache import cache
+import os
+
+
+def is_debug_enabled():
+    """
+    Fast check if debug logging is enabled.
+    Uses cache to avoid DB queries. Returns False immediately if disabled.
+    
+    Returns:
+        bool: True if debug logging enabled, False otherwise
+    """
+    # Check cache first (fastest)
+    cache_key = 'debug_enabled'
+    enabled = cache.get(cache_key)
+    
+    # If we have a cached value (True or False), use it
+    if enabled is not None:
+        return bool(enabled)
+    
+    # Cache miss - check database
+    try:
+        from igny8_core.business.system.models import DebugConfiguration
+        config = DebugConfiguration.get_config()
+        enabled = config.enable_debug_logging
+        
+        # Cache the actual boolean value
+        cache.set(cache_key, enabled, 60)  # Cache for 1 minute
+        
+        return bool(enabled)
+    except Exception as e:
+        # If DB not ready or model doesn't exist, default to False
+        # Cache this to avoid repeated DB errors
+        cache.set(cache_key, False, 10)
+        return False
+
+
+def _should_log_in_this_worker():
+    """
+    Only log in the main worker to avoid duplicate logs.
+    Returns True if this is the first worker or if we should always log.
+    """
+    # Get worker PID - only log from worker with lowest PID to avoid duplicates
+    worker_pid = os.getpid()
+    
+    # Cache the first worker PID that tries to log
+    first_worker = cache.get('debug_first_worker_pid')
+    if first_worker is None:
+        cache.set('debug_first_worker_pid', worker_pid, 300)  # Cache for 5 minutes
+        return True
+    
+    # Only log if we're the first worker
+    return worker_pid == first_worker
+
+
+def debug_log(message, category='general'):
+    """
+    Log a debug message only if debug is enabled.
+    Completely skips processing if debug is disabled.
+    
+    Args:
+        message: Message to log
+        category: Log category (ai_steps, api_requests, db_queries, celery_tasks)
+    """
+    # Fast exit - don't even process the message if debug is disabled
+    if not is_debug_enabled():
+        return
+    
+    # Only log in one worker to avoid duplicates
+    if not _should_log_in_this_worker():
+        return
+    
+    # Check category-specific settings
+    try:
+        from igny8_core.business.system.models import DebugConfiguration
+        config = DebugConfiguration.get_config()
+        
+        # Check category-specific flags
+        if category == 'ai_steps' and not config.log_ai_steps:
+            return
+        if category == 'api_requests' and not config.log_api_requests:
+            return
+        if category == 'db_queries' and not config.log_database_queries:
+            return
+        if category == 'celery_tasks' and not config.log_celery_tasks:
+            return
+    except Exception:
+        pass
+    
+    # Debug is enabled - log to console
+    import sys
+    import datetime
+    timestamp = datetime.datetime.now().strftime("%H:%M:%S")
+    worker_pid = os.getpid()
+    prefix = f"[{timestamp}] [PID:{worker_pid}] [DEBUG:{category.upper()}]"
+    print(f"{prefix} {message}", file=sys.stdout, flush=True)
+
+
+def debug_log_ai_step(step_name, message, **kwargs):
+    """
+    Log an AI execution step only if debug is enabled.
+    Completely skips processing if debug is disabled.
+    
+    Args:
+        step_name: Name of the step (INIT, PREPARE, AI_CALL, etc.)
+        message: Step message
+        **kwargs: Additional context to log
+    """
+    # Fast exit - don't even process if debug is disabled
+    if not is_debug_enabled():
+        return
+    
+    # Only log in one worker to avoid duplicates
+    if not _should_log_in_this_worker():
+        return
+    
+    # Format the message with context
+    context_str = ""
+    if kwargs:
+        context_parts = [f"{k}={v}" for k, v in kwargs.items()]
+        context_str = f" | {', '.join(context_parts)}"
+    
+    full_message = f"[{step_name}] {message}{context_str}"
+    debug_log(full_message, category='ai_steps')