feat(android): Sonos/UPnP queue plays leveled URLs, rendered a track ahead (M464 #5002)

Every URL the Sonos queue loader sends is minted with level=true and
the track's album-play verdict from its neighbours in the queue; the
server returns the plain stream when leveling is off or changes
nothing. The playing track and the one after it are rendered ahead,
and each time the renderer moves on, the next is.

Server: a mint no longer prerenders on its own. A queue load mints
every track, which would have started an ffmpeg render per track at
once. The request now carries prerender, and at most two prerenders
run at a time; past that they are dropped, since a fetch renders on
demand anyway.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/cast_token.go

@@ -26,6 +26,11 @@ type castTokenRequest struct {
 	// client holding the queue knows it.
 	Level   bool `json:"level,omitempty"`
 	AsAlbum bool `json:"asAlbum,omitempty"`
+	// Prerender says the speaker will fetch this track soon: the current
+	// track at a queue load, or the next one as it starts. Only those are
+	// rendered ahead; a queue load mints every track and renders none of
+	// the rest until the speaker asks.
+	Prerender bool `json:"prerender,omitempty"`
 }
 
 type castTokenResponse struct {
@@ -138,11 +143,11 @@ func (h *handlers) handleCastStreamToken(w http.ResponseWriter, r *http.Request)
 			path = leveledStreamPath(trackUUID) + leveledQuery(g, token, exp)
 			mime = "audio/flac"
 			leveled = true
-			// The speaker fetches the URL shortly; start rendering now so
-			// the fetch finds the file ready.
-			h.leveled.Prerender(library.LeveledSource{
-				TrackID: req.TrackID, Path: track.FilePath, DurationMs: track.DurationMs,
-			}, g)
+			if req.Prerender {
+				h.leveled.Prerender(library.LeveledSource{
+					TrackID: req.TrackID, Path: track.FilePath, DurationMs: track.DurationMs,
+				}, g)
+			}
 		}
 	}