Determine if ct_related() endpoint will remain

## Context

Inline TODO at `R/ct_related.R:1` — follow up on whether the related-substances API endpoint is permanent or may be removed. If it's being deprecated, the function may need to be marked as defunct.

## Tasks

- [ ] Check with EPA/CompTox team on endpoint status
- [ ] If permanent, promote lifecycle accordingly
- [ ] If deprecated, mark function as `.Defunct()`