[eval] Scouter write path for eval scenarios and results

[thorrester]

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)