Per-(client, action) usage rollup for the YYYY-MM period.

Reports OAuth-token CALL volume from agent_usage. Revenue is reported as unmetered (null plus an explicit marker), not as 0: agent_usage.revenue_micros has no writer anywhere in the app — check_and_track_usage only ever increments count — so for as long as this route summed it, total_revenue_micros was a confident $0.00 with nothing behind it. There is no truthful source to swap in: x402_payments carries a payer wallet but no OAuth client_id, and ledger_entries.agent_id points at API-key agents, a different population. Until a schema links a settlement to an OAuth client, per-client revenue cannot be stated and this route says so.