Implement Redis caching for SSE notifications and session state

[rsalus]

Context

PostgreSQL persistence is being implemented for WorkItems and PatientRegistry. Redis is already used for AnalysisResultStore caching.

Scope

Future work to consider:

1. SSE Notification Pub/Sub

   • Replace in-memory Channel<Notification> with Redis pub/sub
   • Enables multi-instance deployments with shared notification stream

2. Session Affinity (if needed)

   • Consider Redis for load-balanced session state
   • May not be needed if using sticky sessions

3. Rate Limiting (optional)

   • Redis-backed rate limiting for API endpoints

Not In Scope

• AnalysisResultStore - already uses Redis (no changes needed)

Acceptance Criteria

• SSE notifications work across multiple Gateway instances
• No data loss when instances scale up/down

Related

• Design: docs/designs/2026-02-02-observability-persistence-coverage.md