Rate-limit and usage-meter entries now always include severity and is_active

What

• The rate-limits API response schema gained two new fields on each limit entry: severity and is_active, alongside the existing group, percent, resets_at, and scope fields.
• A related status-meter schema, used for usage/limit meters more broadly, changed severity and is_active from optional and nullable fields to required ones, with an updated description clarifying that every row's severity and active status comes from the server, not something a client computes itself.

Why

This ensures any client displaying usage or rate-limit information always gets a definitive severity level and active/inactive status straight from the server, instead of having to infer it or handle missing values.