As we don't have an API to upload files and create playlists yet, we
used the convention "tag uid as directory name" to allow testing
playback. With the introduction of the database in #39 "Add playlist db"
a builddb() function to initialize the database from the tag directories
was added, but it is not automatically called. To make the developer
experience more ergonomical, add a check for that automatically runs
builddb() when no database exists.
Signed-off-by: Matthias Blankertz <matthias@blankertz.org>
