enclosure_tags: set genre to podcast based on tag

[lemon24]

...makes things easier to find.

The genre seems to come from an enum:

• https://en.wikipedia.org/wiki/List_of_ID3v1_genres
• https://mutagen.readthedocs.io/en/latest/api/id3_frames.html#mutagen.id3.TCON
• https://mutagen-specs.readthedocs.io/en/latest/id3/id3v2.4.0-frames.html#tcon

It may also be possible to specify more than one genre, but podcast should be the first:

• https://forum.minimserver.com/showthread.php?tid=2575
• https://code.videolan.org/videolan/medialibrary/-/issues/463
• https://trac.ffmpeg.org/ticket/6949