BIO-018: the store was a process-global dictionary keyed on the client-supplied
Idempotency-Key alone, so one caller could replay another caller's key and
receive their cached response. The key is now scoped with the caller SubjectId.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
# Conflicts:
# docs/project/refactor-backlog-setup/refactor-backlog/99-backlog.md
# libs/shared/docs/behaviour-spec.mdx
eho
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