[genai-ui] OpsML proxy routes for Scouter GenAI aggregate endpoints #389

Part of #381

What

New OpsML server routes that proxy Scouter bifrost's GenAI semantic attribute endpoints.

• Identify which Scouter endpoints expose GenAI aggregates: timeseries and per-span aggregates
• Add proxy routes under GET /opsml/api/genai/metrics/* in opsml-server
• Response types in opsml-types contracts
• Auth-protected, follows standard handler conventions
• SvelteKit API client updated in opsmlClient.ts

These routes are the dependency for both UI issues in this epic.