Plan limits and current usage for this account

Returns plan { name, max_domains (null = unlimited), min_frequency, allowed_frequencies, history_days, ai_reviews_enabled, manual_ai_reviews_per_month }, usage { domains_used, manual_ai_reviews_used_this_month, manual_ai_reviews_remaining, manual_ai_reviews_reset_at, api_requests_this_period, api_monthly_limit, api_period_resets_at }, and subscription { status, current_period_end, trial_end }.