feat: add configurable log retention policy (v3.14.0)

[kunalnagar]

Summary

• Adds two independent log retention axes: Max Log Count (cap by row count) and Max Log Age (cap by age in days). Both default to 0 (disabled) — zero behaviour change for existing users.
• Daily WP-Cron event (custom_404_pro_prune_logs) runs both passes automatically. Scheduled on activation, unscheduled on deactivation and uninstall.
• On-demand Prune Logs Now button on the Logs page for immediate cleanup.
• Log count stat and a 90%-threshold warning notice displayed on the Logs page.
• Two new settings fields in the General tab with absint(wp_unslash()) sanitization.

Test plan

• Unit tests: composer test — 36/36 pass
• Integration tests: composer test:integration — 29/29 pass (5 new prune tests)
• Verify Max Log Count = 5 with 6+ rows → Prune Now deletes the oldest row
• Verify Max Log Age = 1 with a row inserted yesterday → daily cron or Prune Now removes it
• Verify warning notice appears on Logs page when count ≥ 90% of the cap
• Verify deactivation unschedules the cron event (wp cron event list)
• Verify uninstall removes the cron hook and all plugin data cleanly