[eval] Scouter write path for eval scenarios and results

Part of #380
Depends on: #382

## What

Server-side logic to write eval scenarios and results to Scouter via bifrost.

- New route group `GET/POST /opsml/api/eval/*` under auth middleware in `opsml-server`
- Register endpoint: accepts `EvalScenario`, writes to Scouter, stores `EvalRegistryRecord` in OpsML DB
- Results endpoint: accepts eval run output, posts to Scouter
- Scouter write failures should return 503 with a clear error — no silent partial writes
- Follow existing handler conventions (`AppState`, `UserPermissions`, `AuditContext`)