Fetches the latest selfie verification session for a UBO and, optionally, generates or regenerates a verification link.

Action behavior:

• no action: return the current verification-session state without creating a new link.
• GENERATE: create the first verification link only when no previous selfie session exists.
• REGENERATE: create a fresh verification link only when the latest session is DECLINED or EXPIRED.

Response behavior:

• latestSession: the most recent selfie verification session, or null if none exists.
• previousSessions: older selfie verification sessions in reverse chronological order.