Turn a vague ticket into an implementation plan

Here's the ticket: {{paste}}. Here's the relevant code: {{paste}}.

Produce an implementation plan:
1. Restate the requirement precisely, and list every ambiguity you had to resolve with an assumption. Mark each assumption as safe or needs-confirmation.
2. The questions I must answer before starting — only the ones that would change the design.
3. Files to change, and what changes in each.
4. The plan as ordered commits, each independently reviewable and each leaving main working.
5. Edge cases the ticket ignores.
6. What could break elsewhere — callers, jobs, caches, clients on old versions.
7. Migration and rollout: flag needed, backfill needed, rollback path.
8. Tests per commit.
9. Estimate as a range with the reason for the spread, and name the unknown that could triple it.

Don't write the implementation yet.