action_tidy_research took an `owner` argument and never used it. Any user's
scheduled tidy task swept data/deep_research globally, unlinking every empty
or unparseable file regardless of who owned it.
A broken file has no readable owner stamp, so it cannot be matched against
`owner` the way _find_owned_research_path does, which is why the HTTP path and
manage_research already treat parse failure as not-owned. Clearing one is a
privileged act rather than an ownership one, so gate it on the canonical
owner_is_admin_or_single_user helper: admins and the single-user operator keep
the janitor, a regular user does not, and neither does the pre-setup window
before an admin exists.
Returns before the directory glob rather than filtering inside the loop, so a
denied run reports why instead of reporting "none broken" over files it never
inspected. That reason string surfaces in Activity as a skipped row.
