- New src/packages/cache.rs module with PackageCacheState, stale detection,
state persistence, 404 retry logic
- Add refresh_package_cache() and last_cache_update() to PackageManagerBackend
trait, implemented on all 5 backends (APT, DNF, YUM, APK, Pacman)
- Health check now reports last_cache_update and cache_status fields,
triggers cache refresh if stale (>4h), returns degraded on failure
- Patch apply jobs now force cache refresh before applying patches,
with 404/fetch error retry (1 retry after cache refresh)
- Cache state persists to /var/lib/linux_patch_api/state/cache.json
- Version bump to 1.1.17
- Update ARCHITECTURE.md and REQUIREMENTS.md (FR-007)
Closes: #2
