Members asked to see how the co-op is doing, not just their own usage. The
series was admin-only; now the same query serves both surfaces.
- metrics_series(days) holds the query once; /admin/metrics/series and the new
/broker/metrics both call it (no second copy to drift).
- /broker/metrics is broker-authenticated and returns AGGREGATE, NON-PERSONAL
figures only: co-op size, requests served, spend. Same class as
/broker/model-leaderboard, and the same rule — no per-member figure passes
through the broker.