Returns active subscription counts grouped by plan name for the period.Request: Bearer token (admin)Query:period - one of: last_7_days, monthly, last_3_months, last_6_months, this_year, previous_year (no daily)Success:200 - data.period plus data.distribution[] (each: plan_name, count).Errors:401 - invalid token | 403 - not admin | 422 - invalid period