fix(sharkey): create media dir via tmpfiles, drop ReadWritePaths
The nixpkgs sharkey module bind-mounts mediaDirectory into the service
namespace; if the path doesn't exist the unit fails with NAMESPACE (226).
Create it via systemd.tmpfiles.rules (owned sharkey:sharkey) instead of
relying on ReadWritePaths, which conflicted with the module's sandboxing.
Also strip the now-redundant nixos-rebuild prompt from the migration script.