Partial update. Keys omitted from the request body are left unchanged; send a key with a
null, empty, or whitespace-only value to clear it. Prefer this over
update-journal-entry-metadata when setting or clearing individual keys, as it cannot disturb
keys you did not name. Archived keys may only be cleared, never changed. Every key must
name a definition assigned to journal entries: an unknown key, or one assigned only to
another entity type, fails the request with a 400. Keys are never silently ignored. The
write is atomic: if any key fails validation, nothing is written. Metadata changes apply
immediately, without approval, and are allowed in closed periods.