afs: Switch to sha256 and change default database path.

afs: Switch to sha256 and change default database path.

This switches the hash column of the audio file table to use sha256
rather than sha1. Due to the preparations implemented in the previous
patches this is a mechanical conversion that could have been performed
by a script.

The only non-trivial thing is the rename of the audio file table
directory, which changes from "audio_files" to "audio-files" because
this is the name the conversion script introduced in the previous
commit picked.