[IC-094] Add zero-downtime migration and deployment health gates.

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Outcome

Add zero-downtime migration and deployment health gates.

Acceptance checks

• Backward-compatible rollout sequence is documented/tested; readiness drains workers; rollback does not corrupt in-flight work.

Dependencies

IC-027, IC-089

Planning metadata

• Priority: P1
• Milestone: Milestone 9 — Operations and pilot readiness

Definition of done

• Automated tests cover the change where practical.
• Tenant/environment isolation and failure paths were reviewed.
• Logs and errors contain no secrets, PII, documents, or biometric data.
• Relevant API, architecture, operations, and user documentation is updated.
• The pull request links this issue and lists verification commands.
  Generated from docs/planning/implementation-backlog.md; edit the backlog and rerun the sync to change generated fields.