Manifest Change Detection

HEAD /api/v1/devices/me/manifest with ETag support
  - Create ManifestChecker service for lightweight update checks
  - Send If-None-Match header with stored ETag
  - Handle 304 Not Modified (return :up_to_date)
  - Handle 200 OK (return :update_available with new ETag)
  - Support If-Match for mid-sync validation (detect 412 Precondition Failed)