Profile response detail view shows both version control messages

When viewing an **old revision** of a response in the profile response detail view, both version control banners render at once: the warning "you are viewing an old version" AND the info "this response has N old versions".

## Root cause

* `ResponseHistoryBanner.tsx` renders the two alerts independently; the info alert is not gated on `!response.supersededBy`.
* Backend: `Response.old_versions` filters on `superseded_by=self.current_version`, and `current_version` returns `self.superseded_by or self` — so an old revision's `old_versions` returns the whole sibling set, including itself.

## Fix

Show the info alert only on the current version (and/or exclude `self` from `old_versions`).