slice_and_persist always wrote to get_library_files_dir() while giving the
new row the source folder's id, so slicing a file on a NAS mount produced
a .gcode.3mf that showed up in the right folder in the UI and never
reached the share -- invisible from the web UI, which is why it did not
reproduce.
Resolve the destination from the target folder like uploads (#1112) and
moves already do, set is_external and store the absolute path. Collisions
uniquify to "Model (2).gcode.3mf": a 409 would throw away minutes of CPU
on a routine re-slice, and overwriting a file on someone's NAS is worse.
An external folder that cannot take the file (read-only, unreachable, not
writable) falls back to managed storage rather than discarding the slice,
and reports why on SliceResponse.external_write_fallback -- surfaced as a
warning toast. Silent fallback is what made this bug invisible.
