Phase 0: Add monthly credit replenishment Celery Beat task

- Created billing/tasks.py with replenish_monthly_credits task
- Task runs on first day of each month at midnight
- Adds plan.included_credits to all active accounts
- Creates CreditTransaction records for audit trail
- Configured in celery.py beat_schedule
- Handles errors gracefully and logs all operations

backend/igny8_core/celery.py

@@ -3,6 +3,7 @@ Celery configuration for IGNY8
 """
 import os
 from celery import Celery
+from celery.schedules import crontab
 
 # Set the default Django settings module for the 'celery' program.
 os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'igny8_core.settings')
@@ -18,6 +19,13 @@ app.config_from_object('django.conf:settings', namespace='CELERY')
 # Load task modules from all registered Django apps.
 app.autodiscover_tasks()
 
+# Celery Beat schedule for periodic tasks
+app.conf.beat_schedule = {
+    'replenish-monthly-credits': {
+        'task': 'igny8_core.modules.billing.tasks.replenish_monthly_credits',
+        'schedule': crontab(hour=0, minute=0, day_of_month=1),  # First day of month at midnight
+    },
+}
 
 @app.task(bind=True, ignore_result=True)
 def debug_task(self):