unique_filename produced 'manual (2).pdf'; HouseplanContentView sanitises the
name in the REQUEST too, turning ' (2)' into '_2_', so the file was written and
then 404'd. The same pattern was already in files/migrate, so a rebind that hit
a name collision has been producing dead links. Both use the shared helper now,
with '-2', which round-trips sanitize_filename — asserted.