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/musicbrainz.py

@@ -1,11 +1,14 @@
 from dataclasses import dataclass
 from functools import partial
-from typing import TypedDict
+from typing import Annotated, TypedDict
 from uuid import UUID
 
+from pydantic import Field
+
 from ..tools.types import option_fmap
 
 option_uuid = partial(option_fmap, UUID)
+OptionUUID = Annotated[UUID | None, Field(default=None)]
 
 
 class MusicBrainzTags(TypedDict, total=False):
@@ -44,35 +47,35 @@ class MusicBrainzIds:
 	#: so recording IDs are a much more reliable way to identify a particular
 	#: song.
 	#: https://musicbrainz.org/doc/Recording
-	recording: UUID | None
+	recording: OptionUUID
 
 	#: A MusicBrainz work represents the idea of a particular song or creation
 	#: (it doesn't have to be audio). Each work may have multiple recordings
 	#: (studio versus live, different performers, etc.), with the work ID
 	#: grouping them together.
 	#: https://musicbrainz.org/doc/Work
-	work: UUID | None
+	work: OptionUUID
 
 	#: A MusicBrainz track represents a specific instance of a recording
 	#: appearing as part of some release. For example, if the same song appears
 	#: on both two-CD and four-CD versions of a soundtrack, then it will be
 	#: considered the same "recording" in both cases, but different "tracks".
 	#: https://musicbrainz.org/doc/Track
-	track: UUID | None
+	track: OptionUUID
 
 	#: https://musicbrainz.org/doc/Artist
-	artist: UUID | None
+	artist: OptionUUID
 
 	#: A MusicBrainz release roughly corresponds to an "album", and indeed is
 	#: stored in a tag called MUSICBRAINZ_ALBUMID. The more general name is
 	#: meant to encompass all the different ways music can be released.
 	#: https://musicbrainz.org/doc/Release
-	release: UUID | None
+	release: OptionUUID
 
 	#: Again, the release artist corresponds to an "album artist". These MBIDs
 	#: refer to the same artists in the MusicBrainz database that individual
 	#: recordings' artist MBIDs do.
-	release_artist: UUID | None
+	release_artist: OptionUUID
 
 	#: A MusicBrainz release group roughly corresponds to "all the editions of
 	#: a particular album". For example, if the same album were released on CD,
@@ -81,7 +84,7 @@ class MusicBrainzIds:
 	#: for this tag is relatively new (July 2023) and doesn't seem especially
 	#: reliable, so it might be missing here even if your music has been tagged
 	#: with it. Not sure why. https://musicbrainz.org/doc/Release_Group
-	release_group: UUID | None
+	release_group: OptionUUID
 
 
 def to_brainz(tags: MusicBrainzTags) -> MusicBrainzIds: