File lifetimes without volflag

NOTE:
**please use english, or include an english translation.** aside from that,
all of the below are optional, consider them as inspiration, delete and rewrite at will

**is your feature request related to a problem? Please describe.**
i use copyparty to store both temporary files which i would like to be automatically deleted after some time, and permanent files which should remain indefinitely (unless manually deleted).

**Describe the idea / solution you'd like**
it would be nice if copyparty supported setting lifetimes on individual files, without setting a lifetime on the entire volume.

**Describe any alternatives you've considered**
i tried to achieve this by setting the volume lifetime really far in the future, but that did not work due to #926, and is no longer possible at all since 5d96862c.