fix(backend): keep the saved cover art at full resolution

Search results now carry a small thumbnail, so the artwork downloaded when a
YouTube track is saved would have shrunk with them. The URL is upsized to the
configured artwork resolution right before the download event goes out.

web-player/.env.example

@@ -25,6 +25,17 @@ API_BASE_URL=http://localhost:5111
 # Dev already allows a plain-HTTP issuer. Set this only to allow it in a production build (never with a public issuer).
 # AUTH_ALLOW_INSECURE_HTTP=true
 
+# Optional tuning for the YouTube artwork proxy (/api/youtube/thumbnail),
+# which caches the covers in memory so the browser never hits Google directly.
+# THUMBNAIL_CACHE_MAX_AGE_SECONDS=2592000
+# THUMBNAIL_FAILURE_CACHE_SECONDS=300
+# THUMBNAIL_CACHE_BYTES=134217728
+# THUMBNAIL_MAX_CONCURRENT_FETCHES=4
+# THUMBNAIL_FETCH_TIMEOUT_MS=8000
+# THUMBNAIL_COOLDOWN_SECONDS=30
+# THUMBNAIL_MAX_COOLDOWN_SECONDS=300
+# THUMBNAIL_MAX_SIZE=720
+
 # Where to send presigned S3 PUTs when the public S3 host is not reachable from
 # the server (in Docker: http://seaweedfs:8333). The signed Host header is kept
 # either way. Defaults to the host in the presigned URL.