Persist the collected profile + write a journal-friendly session row.

On success this writes:

• app_students.profile_data (authed) OR app_anonymous_sessions.profile_data (anon), with completed_at stamped inside the JSONB.
• One app_learning_sessions row with module_type='get_to_know', keyed by either student_id or anon_uuid (XOR — enforced by DB CHECK constraint).
• The full conversation transcript into app_messages linked to that session.

For authed users with no Student row yet, we lazy-create one so the data lands in the right table.

Errors are swallowed and reported as saved_to='none' — localStorage is authoritative on the client.