Epic: Usage dashboard: billing-aligned transparency #8183

Usage shows call volume over a rolling window, but customers are billed on a fixed period against a plan limit. There is no way to see where you stand mid-period, and if an organisation gets restricted for overage we block the usage page, so they cannot see what caused it either.

This epic covers the frontend: billing period, plan limit, projection, and keeping the page readable when restricted.

Free plans do not wait for the billing period migration. They are measured over a trailing 30 days and get their own treatment, which collapses into one model if that migration lands.

Out of scope: notification configuration, and overage cost.