How to: file indexing
enable music search, upload-undo, and better dedup

file indexing relies on two database tables, the up2k filetree (`-e2d`) and the metadata tags (`-e2t`), stored in `.hist/up2k.db`. Configuration can be done through arguments, volflags, or a mix of both.

through arguments:
* `-e2d` enables file indexing on upload
* `-e2ds` also scans writable folders for new files on startup
* `-e2dsa` also scans all mounted volumes (including readonly ones)
* `-e2t` enables metadata indexing on upload
* `-e2ts` also scans for tags in all files that don't have tags yet
* `-e2tsr` also deletes all existing tags, doing a full reindex
* `-e2v` verifies file integrity at startup, comparing hashes from the db
* `-e2vu` patches the database with the new hashes from the filesystem
* `-e2vp` panics and kills copyparty instead

the same arguments can be set as volflags, in addition to `d2d`, `d2ds`, `d2t`, `d2ts`, `d2v` for disabling:
* `-v ~/music::r:c,e2ds,e2tsr` does a full reindex of everything on startup
* `-v ~/music::r:c,d2d` disables **all** indexing, even if any `-e2*` are on
* `-v ~/music::r:c,d2t` disables all `-e2t*` (tags), does not affect `-e2d*`
* `-v ~/music::r:c,d2ds` disables on-boot scans; only index new uploads
* `-v ~/music::r:c,d2ts` same except only affecting tags

note:
* upload-times can be displayed in the file listing by enabling the `.up_at` metadata key, either globally with `-e2d -mte +.up_at` or per-volume with volflags `e2d,mte=+.up_at` (will have a ~17% performance impact on directory listings)
  * and file checksums can be shown with global-option `-e2d -mte +w` or volflag `e2d,mte=+w` (always active for users with permission `a`)
* `e2tsr` is probably always overkill, since `e2ds`/`e2dsa` would pick up any file modifications and `e2ts` would then reindex those, unless there is a new copyparty version with new parsers and the release note says otherwise

config file example (these options are recommended btw):

```yaml
[global]
  e2dsa  # scan and index all files in all volumes on startup
  e2ts   # check newly-discovered or uploaded files for media tags
```