Changes to Cope with Low Resource Hosts

v1.13.0: Low-memory hosting fixes — split init/scan and stream file list

Split step_init into init (DB export/upload) and scan (directory walk)
so each runs in its own AJAX cycle. Stream file list directly to
wp_options in 200-file batches during the scan instead of accumulating
50k+ paths in a single array. Add shutdown handler to surface PHP
fatal errors (OOM, timeout) as JSON instead of silent "Status poll
failed".

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>