Process notifications in reverse chronological order (newest first)
- Add queue_file_sort_key() function for consistent sorting
- Sort by priority first (0 before 1), then newest timestamp first
- Updated load_and_process_queued_notifications to use new sorting
- Updated queue reload during processing to maintain sort order
- Most recent notifications now processed first within each priority group
This ensures fresh mentions and replies get immediate attention
while maintaining priority for @cameron.pfiffer.org.
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