v3.8.1 — Cache installation recovery

Fixes cache installation stopping after deleting the last old rule: Cloudflare returns empty rulesets without a rules field, which is now handled correctly.

If all cache rules were manually removed while installation was paused, Resume cache installation now verifies both cache phases are empty and restarts safely. The original backup remains available for download and rollback. Surviving rules retain conflict protection. Recovery does not install security rules or purge caches.

Validated with empty-ruleset migration and rollback, recovery after deleted rules/rulesets, original-backup retention, scope/prerequisite checks, remaining-rule conflicts, canonical policy regressions, and GitHub's full validation suite.