Join Room By Code

Redeem a one-time share code and join its room as the authenticated account.

The joiner is ALWAYS the verified session account (user_<sub>), NEVER the request body — the same anti-spoof invariant as the data-plane /memory/rooms/join, exposed here on the session-auth management plane so the console /join landing can redeem for a signed-in user. The code IS the grant (role baked in at mint), so there is no owner gate — anyone signed in with a valid code may join. Idempotent: an account already a member returns already_member=true without consuming a use. reason maps to HTTP status: not_found -> 404; used/expired/revoked -> 409.