Fixed Backup Location and Updater

v1.5.0: Disk-backed temp dir, updater fixes

Backup temp files now stage in wp-content/yawp-tmp/ instead of /tmp,
which may be RAM-backed (tmpfs) on shared hosting. A ~1 GB archive in
tmpfs exhausted the cgroup memory limit on OVH and crashed the site.

- Use disk-backed wp-content/yawp-tmp/ for backup staging
- Register shutdown function for emergency cleanup on fatal/OOM
- Clean up stale backup temp dirs at start of each run
- Exclude yawp-tmp/ from tar archives
- Fix Update URI (was "false", blocking all update checks)
- Populate no_update transient so auto-updates toggle appears
- Remove aggressive cache clearing that hammered GitHub API
- Increase update cache TTL to 12 hours
- Fix post_install to activate with canonical yawp/yawp.php slug

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