fix: preserve user context in async NowPlaying dispatch

Fixed issue #4787 where plugin scrobblers received an empty username during NowPlaying events. The async worker was passing context.Background() which lost all user information.

Changed nowPlayingEntry to store the full context (with cancellation removed via context.WithoutCancel) and pass it to dispatchNowPlaying. This ensures plugin scrobblers can extract username from the context for authorization checks.

Updated tests to verify username is properly propagated through the async workflow, matching the actual plugin adapter behavior of checking both request.UsernameFrom and request.UserFrom.

core/scrobbler/play_tracker.go

@@ -32,6 +32,7 @@ type Submission struct {
 }
 
 type nowPlayingEntry struct {
+	ctx      context.Context
 	userId   string
 	track    *model.MediaFile
 	position int
@@ -220,15 +221,17 @@ func (p *playTracker) NowPlaying(ctx context.Context, playerId string, playerNam
 	}
 	player, _ := request.PlayerFrom(ctx)
 	if player.ScrobbleEnabled {
-		p.enqueueNowPlaying(playerId, user.ID, mf, position)
+		p.enqueueNowPlaying(ctx, playerId, user.ID, mf, position)
 	}
 	return nil
 }
 
-func (p *playTracker) enqueueNowPlaying(playerId string, userId string, track *model.MediaFile, position int) {
+func (p *playTracker) enqueueNowPlaying(ctx context.Context, playerId string, userId string, track *model.MediaFile, position int) {
 	p.npMu.Lock()
 	defer p.npMu.Unlock()
+	ctx = context.WithoutCancel(ctx) // Prevent cancellation from affecting background processing
 	p.npQueue[playerId] = nowPlayingEntry{
+		ctx:      ctx,
 		userId:   userId,
 		track:    track,
 		position: position,
@@ -267,7 +270,7 @@ func (p *playTracker) nowPlayingWorker() {
 
 		// Process entries without holding lock
 		for _, entry := range entries {
-			p.dispatchNowPlaying(context.Background(), entry.userId, entry.track, entry.position)
+			p.dispatchNowPlaying(entry.ctx, entry.userId, entry.track, entry.position)
 		}
 	}
 }