Groups (stereo pairs of ST10 speakers) were read-only — the GET endpoint
always returned an empty <group/>. Add POST /account/{account}/group,
POST /account/{account}/group/{groupId}, and DELETE
/account/{account}/group/{groupId} with datastore persistence, matching
the API shape observed in soundcork. The GET endpoint now reads live
group state from the datastore.
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>