[Incident][108dev] Intermittent 500 on essays endpoints from SWA frontend

## Summary
A new frontend incident was observed during live demo prep: essays screen failed with 500 responses on APIM routes.

Observed from browser console (SWA):
- GET /api/essays/resources -> 500
- GET /api/essays/essays -> 500
- GET /api/essays/assemblies -> 500

## Scope
- Environment: 108dev
- Source: SWA frontend (
ice-flower-0be65ed0f.1.azurestaticapps.net)
- Impact: Essays page displays no available cases and cannot load data.

## Verification done now
Re-check from command line against same APIM routes:
- Preflight CORS for all three routes -> 200 with Access-Control-Allow-Origin set to SWA origin.
- Functional GET for all three routes -> 200 with payload.

This indicates an intermittent live regression window (transient backend/runtime drift or in-flight deployment state), not a persistent outage at check time.

## Actions tracked
- Live remediation actions and drift context are tracked in #139.
- This issue is opened to track intermittent essays endpoint instability specifically.

## Next steps
- [ ] Add periodic synthetic checks for /api/essays/resources|essays|assemblies from SWA origin.
- [ ] Backport runtime drift protections to IaC/workflow to prevent env resets.
- [ ] Correlate failures with active workflow/deploy windows and revision rollouts.

Related: #139