How to: metadata from audio files
set `-e2t` to index tags on upload

`-mte` decides which tags to index and display in the browser (and also the display order), this can be changed per-volume:
* `-v ~/music::r:c,mte=title,artist` indexes and displays *title* followed by *artist*

if you add/remove a tag from `mte` you will need to run with `-e2tsr` once to rebuild the database, otherwise only new files will be affected

but instead of using `-mte`, `-mth` is a better way to hide tags in the browser: these tags will not be displayed by default, but they still get indexed and become searchable, and users can choose to unhide them in the `[⚙️] config` pane

`-mtm` can be used to add or redefine a metadata mapping, say you have media files with `foo` and `bar` tags and you want them to display as `qux` in the browser (preferring `foo` if both are present), then do `-mtm qux=foo,bar` and now you can `-mte artist,title,qux`

tags that start with a `.` such as `.bpm` and `.dur`(ation) indicate numeric value

see the beautiful mess of a dictionary in [mtag.py](https://github.com/9001/copyparty/blob/hovudstraum/copyparty/mtag.py) for the default mappings (should cover mp3,opus,flac,m4a,wav,aif,)

`--no-mutagen` disables Mutagen and uses FFprobe instead, which...
* is about 20x slower than Mutagen
* catches a few tags that Mutagen doesn't
  * melodic key, video resolution, framerate, pixfmt
* avoids pulling any GPL code into copyparty
* more importantly runs FFprobe on incoming files which is bad if your FFmpeg has a cve

`--mtag-to` sets the tag-scan timeout; very high default (60 sec) to cater for zfs and other randomly-freezing filesystems. Lower values like 10 are usually safe, allowing for faster processing of tricky files