Query Metrics

Bounded batch of metric queries: shared params validated once, per-query params validated against each metric's own catalog entry, a max of 20 queries, and a failing metric embeds its own problem object in results rather than discarding successful siblings or failing the whole request.

Request shape: {"params": {...shared, optional...}, "queries": [{"metric": "...", "params": {...per-query, optional...}}]}. Per-query params override the shared ones on conflict.