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>

config/celery.py

@@ -2,10 +2,13 @@
 Celery configuration for KeepItGoing.
 """
 
+import logging
 import os
 from celery import Celery
 from celery.schedules import crontab
 
+logger = logging.getLogger(__name__)
+
 # Set the default Django settings module
 os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'config.settings')
 
@@ -32,4 +35,4 @@ app.conf.beat_schedule = {
 
 @app.task(bind=True)
 def debug_task(self):
-    print(f'Request: {self.request!r}')
+    logger.debug(f'Request: {self.request!r}')