[bercianor]

Summary

• Add JournalBackfillChange to copy historical audit entries into journal streams without modifying audit history; registration and execution ordering remain outside this change.
• Preserve history order and source payloads, reuse persistence instances by stage, and exclude the backfill change's own audit records.
• Require journal events to be enabled and stop on unsuccessful appends; document that retries with a fresh instance may duplicate earlier entries.

Testing

• Added six unit tests covering feature gating, empty history, payload and ordering preservation, stage-specific persistence reuse, append failures, and retry behavior.
• Tests were not rerun for this PR; automated validation remains pending CI.