Preview Team Code

Return ONLY the team NAME a code points at, so a joiner can confirm which team they're requesting to join before submitting (POST /join). Grants nothing and reveals nothing else — no owner, members, or seat counts; the owner-approval gate still governs actual membership. Any invalid / expired / inactive code returns a uniform 404 so probing a wrong code learns nothing.

MUST stay defined BEFORE /{lab_id} so the static path wins the route match (FastAPI matches in definition order).