docs: record issue 203 live Staging validation

[FinnThePanther]

Summary

Record the successful live Staging validation for #203 after promoting its merged implementation through the existing guarded #202 workflow. All acceptance criteria now have evidence.

Related issue

Closes #203

Changes

• Retain the sanitized exact-deployment promotion record for source 04f8383fe750bec712ced27a1932b82b1eabb292 and deployment 57f17ef7-7b34-4c2f-9272-b8091b1eafad.
• Record independent public health/identity and credential-free CLI/primitive checks in the Staging runbook and contract; complete the implementation plan's live validation step.

Validation

• Exact successful push API validation: run 35189808670, attempt 1, for the promoted SHA.
• One supported [Work]: Establish the controlled V0 Staging deployment flow #202 promotion: all gates SUCCEEDED, final active state ACTIVE, overall SUCCEEDED.
• Canonical public live/ready/identity: HTTP 200, exact approved JSON, application/json, no-store, no redirects.
• Credential-free CLI and reusable preflight passed and captured the promoted SHA/deployment with environment staging. Nine unsafe-origin variants rejected before transport.
• Documentation: ./scripts/doctor.sh required checks and git diff --check passed; reviewed allowlisted JSON record. Optional Dev Container CLI unavailable. No application code changed in this PR.

Risks and considerations

• Observations are point-in-time. Future deployment changes require a fresh preflight.
• Deployment timestamp in the operator promotion record remains optional enrichment; public preflight required neither timestamp nor Railway credentials.
• No configuration changes, live chaos, acceptance/load traffic, vendor probes, recovery, or [Feature]: Convention Simulation & Headless Acceptance #199 orchestration. This PR publishes evidence only.

Checklist

• I reviewed the linked issue and its acceptance criteria.
• I kept this pull request focused on one coherent change.
• I tested or otherwise validated the change.
• I added or updated tests where the change requires them.
• I updated relevant documentation.
• I did not add credentials, private user data, or other secrets.
• I have identified any follow-up work that is outside this pull request.

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📥 Commits

Reviewing files that changed from the base of the PR and between 04f8383 and 34dc9a3.

📒 Files selected for processing (4)

• docs/development/staging-deployments/57f17ef7-7b34-4c2f-9272-b8091b1eafad.json
• docs/development/staging.md
• docs/specs/2026-09-16-backend-health-environment-safety-implementation-plan.md
• docs/specs/2026-09-16-backend-health-environment-safety.md

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