A member of one trip could point a file at a reservation, place or
day-assignment belonging to another, private trip — on upload, on a
metadata update, or through the file-link endpoint. The reservation join
in the file list and the links list then returned that trip's reservation
title, disclosing it across the trip boundary and letting an attacker
enumerate foreign reservation titles by their id.
The file already had to belong to the caller's trip; now the linked
reservation/place/assignment must too. findForeignLinkTarget checks each
supplied id against the trip (assignments via day -> trip) and the upload,
update and link handlers reject a cross-trip reference with 400 before it
is stored. Same-trip links and clearing a link are unchanged.
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