ProfileGroupItem missing join_policy in frontend type

## Bug Backend ProfileGroupItem (used in OrganizationProfile.groups) includes join_policy: JoinPolicy but frontend type omits it. ## Impact Cannot display open/closed/invite-only badge on ring cards in the org profile page. Users cannot see join requirements before clicking. ## Fix Add join_policy: JoinPolicy to frontend ProfileGroupItem type. ## Found by API contract audit (2026-03-19)