Accepting an invite adds a member to the community but seats them in no channels (including default channels)

### Summary

A user who accepts an invite becomes a community member but is not added to any channel — including default/starter channels. They land in an empty space with nothing to read and no obvious next action.

### Steps to reproduce

- Create an invite for a community that has one or more default/starter channels.
- Accept the invite as a new identity.
- Observe: the account is now a community member (visible to the owner in the Members/Invites UI and present in `relay_members`), but has zero channel memberships (no `channel_members` rows), so no channel content is visible.

### Impact

Combined with the agent-roster gap (#3326), new members are simultaneously invisible to greeter agents and unable to see any content. On our launch day (14-member, self-hosted community) several accepted members sat in the community seated in no channel. Our workaround was to have an agent run `channels add-member` for every unseated member on a schedule — which only became possible after building the roster reader described in #3326.

### Suggested fix

Seat invite-accepting members into the community's default channels on join, or expose the intended mechanism for doing so (and document it), so a new member lands somewhere with content rather than an empty space.

### Environment

Self-hosted `deploy/compose` derivative, `ghcr.io/block/buzz:main`, relay behind Caddy with valid TLS; desktop app + iOS app. Community launched July 2026, 14 members.