Get the current challenge status for the authenticated user. If no challenge exists, checks eligibility for online-eligible challenges (onboarding, monthly) and creates one if eligible. Reengagement challenges are NOT created here (batch job only).

Returns: - If no challenge: { "has_challenge": false } - If pending challenge: { "has_challenge": true, "status": "pending", "challenge": {...} } - If active challenge: { "has_challenge": true, "status": "active", "challenge": {...}, "progress": {...} } - If completed/expired: { "has_challenge": false } (or optionally include for history)