Adds a Reopen button on the session-detail view for finished sessions.
POSTs to /dashboard/healthchecks/{id}/reopen with a per-session CSRF
token; on success clears finished_at and redirects back to the session.
Also tightens Controllers/Healthchecks::update so a PUT with no
finished_at key is a no-op instead of silently setting it to NULL —
otherwise an empty PUT would reopen any completed healthcheck.
After reopening, the WP-side Session::resume($id) flow pulls the now-
active session back into an editable local session on the client site.
Steve Hanlon
d4f643528d