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)
HEAD /api/v1/devices/me/manifest with ETag support