fix: add package cache refresh before apply and on health check

- 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

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "linux-patch-api"
-version = "1.1.16"
+version = "1.1.17"
 edition = "2021"
 authors = ["Echo <echo@moon-dragon.us>"]
 description = "Secure remote package management API for Linux systems"