Document runtime roles and health checks

[cursor]

Docs added/updated

• Updated docs/01-deployment-infrastructure/SCALING_POWERHOUSE.md with current RELEASE_ROLE behavior, Redis-backed PubSub/Hammer activation via REDIS_URL, segmented Oban queues, and production gzip status.
• Updated docs/01-deployment-infrastructure/OBSERVABILITY_MANUAL.md with role-specific /health response examples and status semantics.
• Updated docs/06-admin-system-operations/POST_DEPLOY_SMOKE_CHECKLIST.md to verify expected release_role after deploys.

Codepaths covered

• studentplug/lib/studentplug/release_role.ex
• studentplug/lib/studentplug/application.ex
• studentplug/lib/studentplug_web/controllers/health_controller.ex
• studentplug/config/runtime.exs
• studentplug/config/config.exs
• studentplug/config/prod.exs

Key knowledge gaps addressed

• Clarified that single-service production uses unset/all, while split deployments should use web and worker.
• Documented that Redis behavior is runtime-gated by REDIS_URL and must be present on all nodes before horizontal web scaling.
• Replaced stale single-queue/worker-health assumptions with the current queue and /health contracts.

Verification

• git diff --check