Fix Critical Security Issue: Replace print() with proper logging

Replaced all debug print() statements with proper logging framework:
- tasks/views.py: 5 print statements → logger.debug()
- sync/views.py: 15 print statements → logger.debug()
- config/celery.py: 1 print statement → logger.debug()
- notifications/tasks.py: 2 print statements → logger.error()

Retained print() in settings files (development.py, selfhosted.py) as they are appropriate warnings to stderr for configuration issues.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

notifications/tasks.py

@@ -2,10 +2,13 @@
 Celery tasks for notifications.
 """
 
+import logging
 from celery import shared_task
 from django.utils import timezone
 from datetime import timedelta
 
+logger = logging.getLogger(__name__)
+
 
 @shared_task
 def send_due_reminders():
@@ -157,7 +160,7 @@ def send_fcm_notification(token, title, body, data=None):
 
     except Exception as e:
         # Log error but don't fail
-        print(f"FCM notification failed: {e}")
+        logger.error(f"FCM notification failed: {e}")
 
 
 @shared_task
@@ -187,7 +190,7 @@ def send_web_push_notification(subscription_info, title, body, data=None):
         )
 
     except Exception as e:
-        print(f"Web push notification failed: {e}")
+        logger.error(f"Web push notification failed: {e}")
 
 
 @shared_task