TagSubjectType is the kind of entity a tag is attached to. Mirrors the DB
tag_subject_type enum and keys the polymorphic tag-subject RPCs that replace
the per-subject tag-attach RPCs.
Deliberately distinct from AccessRelationshipSubjectType: the two overlap
but diverge (tags add workflow/guidance; owners add team) and back separate
DB enums. Unifying them would trade enum type-safety for runtime subset
validation.