Schedule Daily Notifications

Daily cron job endpoint to schedule notifications for all users. This should be called once per day (e.g., at midnight UTC).

For each user with push tokens:

• Randomly selects a time slot (morning/noon/night)
• Schedules a notification with content appropriate for that time
• Respects user's timezone for scheduling

Tasks are queued in Redis and processed by Celery workers. Use days_ahead to override the default 2-day lead time.

Requires x-api-key header for authentication.