Carry a guest session's call data onto the account that just logged in.

Nancy, 2026-08-26: "when a student does a whole class as a guest and then creates an account or joins after the class finishes, does their transcript from that class get carried over?"

Signing UP already did this — _transfer_anonymous_call_data runs on registration and on the OAuth merge. LOGGING IN did not, and for a school that is the common half: a student who already has a FluenTea account joins the class by code without logging in, does the whole hour as a guest, and then signs in. Their transcript, their presence in that room and therefore their attendance all stayed on the throwaway guest row.

Two guards, because this moves data between accounts:

• the source must actually BE anonymous — never another real account;
• and recent. A stale anonymous_user_id in a shared browser's storage must not hand a stranger's guest session to whoever logs in next week.