[Epic] GenAI Semantic Metrics UI

## Overview

Surface GenAI semantic attributes extracted from OTEL traces in the OpsML UI. Scouter now extracts GenAI attributes from spans (token counts, model name, latency, etc.) and exposes aggregate + timeseries endpoints via bifrost. This epic wires those into the trace detail view and agent service dashboard.

## Scope

- OpsML server proxy routes for the new Scouter GenAI aggregate endpoints
- Trace/span detail: new metrics tab showing per-span GenAI attributes
- Agent service dashboard: new GenAI metrics subnav with timeseries view

## Sub-issues

- [ ] OpsML proxy routes for Scouter GenAI aggregate endpoints
- [ ] Trace/span detail: GenAI metrics tab
- [ ] Agent service dashboard: GenAI metrics subnav

## Dependency order

`proxy routes` → `trace metrics tab` + `service dashboard` (parallel)

Start with the OpsML server proxy routes for Scouter’s GenAI aggregate endpoints, then trace how those results feed the trace/span detail view and agent service dashboard. The work is done when the proxy routes, per-span GenAI metrics tab, and dashboard GenAI timeseries subnav are all wired in the stated dependency order.

Written by the indexing model from the issue text.