preview_url was assembled from a /api/view link whose type was chosen by
matching the asset's tags against "input" then "output". Anything written
anywhere else - temp above all, where preview nodes put their images - fell
off the end of that chain and came back with no preview at all. Tags are
user-editable, so removing one also silently destroyed the URL.
Derive the URL from where the file actually sits instead. That covers every
root /api/view serves, temp included, and no longer depends on tags or on a
filename in user_metadata. A file outside those roots, or content no client
can render from its own bytes, gets no preview URL rather than one that
cannot work.
Nominated previews are resolved a page at a time rather than per row, so a
list costs one extra query however long it is. A preview that is soft-deleted
or not visible to the caller drops out of that lookup and is no longer
advertised.
Simon Pinfold
Luke Mino-Altherr