API and private thread updates

Several changes improved API coverage and private thread behavior.

B2 API additions

The B2 API now includes more complete discussion and poll operations.

API clients can work with:

Comment support includes editing and soft deletion.

User API behavior was also clarified by separating redaction and deletion paths.

A groups endpoint was added to the B2 API, making it easier for integrations to discover or work with group records.

The B3 API now exposes whether a user is an admin where that information is needed by clients.

Invitees on private threads are no longer automatically granted admin access.

This keeps private-thread permissions narrower: being invited into a private thread does not imply permission to administer it.