Per-API-key request volume, error rate, and recent activity

Joins `api_keys` with `api_key_request_counters` to produce one row per non-revoked key, summed over the [since, until] window. Default sort: -requests (total requests DESC). The window defaults to the trailing 24h and is capped at 30 days; `limit` is clamped to [1, 500]. Every call writes `admin.keys-usage.viewed` to the audit log. Sortable: requests, errorRate, lastUsedAt.