stream: scrobble listened tracks to ListenBrainz

The web player decides when a track counts as listened (caught near its
start and heard to ~90%, capped at 4 min) and triggers POST /scrobble.
The token stays server-side (RADIEO_LISTENBRAINZ_TOKEN), submitting the
listen with the canonical MusicBrainz MBID when available. Each airing is
deduplicated so multiple tabs submit it once.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -113,7 +113,25 @@ ListenBrainz only *names* tracks; each suggestion is resolved to a concrete file
 MusicBrainz MBID the feed already carries. Leave the variable empty to disable.
 A local file path under `config/` also works for testing.
 
-### 5. Run it
+### 5. Scrobbling to ListenBrainz (optional)
+
+Set `RADIEO_LISTENBRAINZ_TOKEN` (in `.env`) to your ListenBrainz *user token*
+(from <https://listenbrainz.org/settings/>) to scrobble what you listen to:
+
+```
+RADIEO_LISTENBRAINZ_TOKEN=<your-user-token>
+```
+
+The web player decides when a track counts as *listened* — caught near its
+start (server position < 10 s) and actually heard to ~90 % of its length (capped
+at 4 min, ListenBrainz's own rule) — then triggers `POST /scrobble`. The token
+stays server-side (never exposed to the browser); the stream submits the listen,
+using the canonical MusicBrainz MBID when the canonicalizer found one. Only
+listeners on the web page scrobble (external players like VLC don't), and each
+airing is submitted once even with several tabs open. Leave the variable empty
+to disable.
+
+### 6. Run it
 
 ```sh
 docker compose up -d