Epic: Usage dashboard: billing-aligned transparency

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.

The issue names no files, tests, or entry points. Start by locating the frontend usage dashboard, then trace how billing periods, plan limits, projections, free-plan trailing-30-day usage, and restricted-page rendering are represented; done means these states are readable and aligned with the stated billing model.

Written by the indexing model from the issue text.