Current usage and tier limits for the authenticated API key

Returns the caller's pricing tier, per-window limits, current usage, remaining quota, billing period boundary, and entitlement flags. current_usage counters are recorded for every tier — including unlimited (enterprise) keys, whose windows are counted observationally even though no limit is enforced — so they are safe to build usage-based alerting on. tier_limits.alert_subscriptions / current_usage.alert_subscriptions_count refer to alert subscriptions managed via the /v1/alerts endpoints (POST /v1/alerts/subscriptions, GET /v1/alerts/subscriptions, DELETE /v1/alerts/subscriptions/{id}) — there is no /v1/subscriptions endpoint. Not rate-limited — safe to poll from the developer portal.