Introduce new WebSockets receiver impl

When enabled, this new receiver will spin up a local WebSockets server
and will send the currently playing song information to any clients that
connect. It's designed with Übersicht in mind, since WebSockets is the
easiest way to efficiently push events into an Übersicht widget, but
I'm sure it'd work for a variety of other purposes too.

Currently the socket is only used in one direction, pushing the current
song info from server to client, but I'll probably extend it to support
sending MPD commands from WebSockets clients as well.

src/mpd_now_playable/song/song.py

@@ -2,6 +2,7 @@ from dataclasses import dataclass
 from enum import StrEnum
 from pathlib import Path
 
+from ..tools.schema.define import schema
 from .artwork import Artwork
 from .musicbrainz import MusicBrainzIds
 
@@ -12,21 +13,69 @@ class PlaybackState(StrEnum):
 	stop = "stop"
 
 
+@schema("https://cdn.00dani.me/m/schemata/mpd-now-playable/song-v1.json")
 @dataclass(slots=True)
 class Song:
+	#: Whether MPD is currently playing, paused, or stopped. Pretty simple.
 	state: PlaybackState
+
+	#: The zero-based index of the current song in MPD's queue.
 	queue_index: int
+	#: The total length of MPD's queue - the last song in the queue will have
+	#: the index one less than this, since queue indices are zero-based.
 	queue_length: int
+
+	#: The relative path to the current song inside the music directory. MPD
+	#: itself uses this path as a stable identifier for the audio file in many
+	#: places, so you can safely do the same.
 	file: Path
+
+	#: The song's title, if it's been tagged with one. Currently only one title
+	#: is supported, since it doesn't make a lot of sense to tag a single audio
+	#: file with multiple titles.
 	title: str | None
+
+	#: The song's artists. Will be an empty list if the song has not been
+	#: tagged with an artist, and may contain multiple values if the song has
+	#: been tagged with several artists.
 	artist: list[str]
+	#: The song's composers. Again, this is permitted to be multivalued.
 	composer: list[str]
+	#: The name of the song's containing album, which may be multivalued.
 	album: list[str]
+	#: The album's artists. This is often used to group together songs from a
+	#: single album that featured different artists.
 	album_artist: list[str]
+
+	#: The track number the song has on its album. This is usually one-based,
+	#: but it's just an arbitrary audio tag so a particular album might start
+	#: at zero or do something weird with it.
 	track: int | None
+
+	#: The disc number of the song on its album. As with the track number, this
+	#: is usually one-based, but it doesn't have to be.
 	disc: int | None
+
+	#: The song's genre or genres. These are completely arbitrary descriptions
+	#: and don't follow any particular standard.
 	genre: list[str]
+
+	#: The song's duration as read from its tags, measured in seconds.
+	#: Fractional seconds are allowed.
 	duration: float
+
+	#: How far into the song MPD is, measured in seconds. Fractional seconds
+	#: are allowed. This is usually going to be less than or equal to the
+	#: song's duration, but because the duration is tagged as metadata and this
+	#: value represents the actual elapsed time, it might go higher if the
+	#: song's duration tag is inaccurate.
 	elapsed: float
+
+	#: The song's cover art, if it has any - the art will be available as bytes
+	#: if present, ready to be displayed directly by receivers.
 	art: Artwork
+
+	#: The MusicBrainz IDs associated with the song and with its artist and
+	#: album, which if present are an extremely accurate way to identify a
+	#: given song. They're not always present, though.
 	musicbrainz: MusicBrainzIds