feat(release): add phase10 tests, smoke load script, release checklist, and onboarding simulator flow

Backend/docs/release-checklist.md

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+# Release Checklist (Phase 10)
+
+## Functional verification
+- [ ] Device registration and heartbeat flow works for both camera and client roles.
+- [ ] Camera-client linking and command dispatch/ack flows work end-to-end.
+- [ ] Motion start/end events are persisted and notify linked clients.
+- [ ] On-demand stream request/accept/end flow works with media credentials.
+- [ ] Recording placeholder creation, finalize, and download URL generation work.
+- [ ] Push fallback queue delivers notifications when realtime channel is offline.
+
+## Security and reliability
+- [ ] Rate limit thresholds validated in staging.
+- [ ] Audit logs capture stream/event/recording sensitive operations.
+- [ ] Env vars present: auth base URL, media provider, database, MinIO, secrets.
+- [ ] Worker loops running: recordings worker and push worker.
+
+## Observability
+- [ ] `/ops/live` and `/ops/ready` healthy.
+- [ ] `/ops/metrics` reports request counters and status counters.
+- [ ] Structured request logs are visible in deployment log sink.
+
+## Pre-release load and tests
+- [ ] Unit tests pass (`bun test`).
+- [ ] Smoke load script passes (`bun run load:smoke`).
+- [ ] Manual run with simulator in two tabs confirms camera+client happy path.
+
+## Rollout
+- [ ] Deploy to staging and run checklist above.
+- [ ] Rollout production gradually and watch error/status metrics.
+- [ ] Keep rollback procedure ready (previous image/version + migration compatibility).