Add .is_not_found() for Store errors, wire up Object and File storage to properly set NotFound
Allow skipping files that are not found during store migration
By default, cached media should only stick around for 7 days, however
The timeout is reset every time media is accessed, so only obscure
cached media will be flushed from the cache. This '7 days' number is
configurable through the commandline run options as --media-cache-duration
and in the pict-rs.toml file as [media] cache_duration