refactor: make NowPlaying dispatch asynchronous with worker pool (#4757)

* feat: make NowPlaying dispatch asynchronous with worker pool

Implemented asynchronous NowPlaying dispatch using a queue worker pattern similar to cacheWarmer. Instead of dispatching NowPlaying updates synchronously during the HTTP request, they are now queued and processed by background workers at controlled intervals.

Key changes:
- Added nowPlayingEntry struct to represent queued entries
- Added npQueue map (keyed by playerId), npMu mutex, and npSignal channel to playTracker
- Implemented enqueueNowPlaying() to add entries to the queue
- Implemented nowPlayingWorker() that polls every 100ms, drains queue, and processes entries
- Changed NowPlaying() to queue dispatch instead of calling synchronously
- Renamed dispatchNowPlaying() to dispatchNowPlayingAsync() and updated it to use background context

Benefits:
- HTTP handlers return immediately without waiting for scrobbler responses
- Deduplication by key: rapid calls (seeking) only dispatch latest state
- Fire-and-forget: one-shot attempts with logged failures
- Backpressure-free: worker processes at its own pace
- Tests updated to use Eventually() assertions for async dispatch

Signed-off-by: Deluan <deluan@navidrome.org>

* fix(play_tracker): increase timeout duration for signal handling

Signed-off-by: Deluan <deluan@navidrome.org>

* refactor(play_tracker): simplify queue processing by directly assigning entries

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

core/scrobbler/buffered_scrobbler_test.go

@@ -38,9 +38,9 @@ var _ = Describe("BufferedScrobbler", func() {
 	It("forwards NowPlaying calls", func() {
 		track := &model.MediaFile{ID: "123", Title: "Test Track"}
 		Expect(bs.NowPlaying(ctx, "user1", track, 0)).To(Succeed())
-		Expect(scr.NowPlayingCalled).To(BeTrue())
-		Expect(scr.UserID).To(Equal("user1"))
-		Expect(scr.Track).To(Equal(track))
+		Expect(scr.GetNowPlayingCalled()).To(BeTrue())
+		Expect(scr.GetUserID()).To(Equal("user1"))
+		Expect(scr.GetTrack()).To(Equal(track))
 	})
 
 	It("enqueues scrobbles to buffer", func() {