Aggregate MCP tools/call audit rows over the last days.

Shape::

{
  "window_days": <int>,
  "total_calls": <int>,
  "by_tool": [
    {"tool_name", "calls", "errors", "error_rate",
     "p50_latency_ms", "p95_latency_ms"},
    ...
  ],
  "by_source": [
    {"caller_source", "calls"},
    ...
  ],
  "top_callers": [
    {"caller_id", "calls"},
    ...
  ],
}

Returns the empty shape when the audit table is missing (pre-0161 deploys) — the FE renders "no data yet" rather than crashing.