Returns the same composite payload as /active-state but is the canonical read endpoint for the Preview screen. It works for sessions in any status (scheduled / in_progress / completed / etc.) and performs no state transitions or domain-event emissions.

Authorized callers:

• Coaches assigned to the session's group (or co-coaches via cohort_coaches) — full payload as in MED-873.
• Internal staff (internal_admin / internal_user) — added in MED-965 so cohort managers can preview from the Session Content page. The coach-ownership check is skipped; structural guards still apply (must be a group session, modality must be coach-led). Per AC4 of MED-965, coach-specific fields are blanked for these callers: coach_session_notes_html → empty string, coach_session_notes_updated_at → null, leader_notes → empty array, participants → empty array, coach_present → false.

IV1 contract: calling this endpoint MUST NOT mutate session state, advance section index, or notify participants.