Dev: Rekey S3 objects from ISO date file names to ISO date paths

**Describe the task**
We have long lists of flat folders/files following the yyyy-mm-dd ISO naming scheme, however these are hard on the object store to list all at once, as well as visually inspect. We could instead store files based on a yyyy/mm key path scheme.

**Acceptance Criteria**
- [ ] Agree on a key path scheme for dates
- [ ] All existing data is migrated to the new scheme above
- [ ] All new data is stored using the new scheme above

**Additional context**
- Add any other context about the task here.
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The issue names no files, tests, or entry points. First locate the S3 object-writing code and any existing data migration tooling, then confirm the proposed date key scheme with maintainers. Done means existing objects are migrated and new objects consistently use the agreed yyyy/mm path scheme.

Written by the indexing model from the issue text.