fix(tts): play via LOCAL_INTERNET_RADIO; accept app_key at /v1/auth

Root cause of the failed TTS playback: the /speaker notification path
makes the speaker validate the app_key via GET /v1/auth against the
service, which returned 404 -> the speaker reports an invalid app key
(HandleInvalidAppKeyCb) and refuses to play. Our /media/tts hosting was
fine all along (confirmed by a direct GET returning the mp3).

Two fixes:

- TTS speak now plays the synthesized clip as a LOCAL_INTERNET_RADIO
  ContentItem via the /custom/v1/playback proxy (the same mechanism the
  "ding" health check uses), which needs no app_key. New
  buildCustomPlaybackURL helper + tts.Service.BaseURL().
- Add GET /v1/auth -> 200 so the /speaker notification path also works
  (we're the cloud replacement; a 404 there is read as "invalid app
  key"). Includes a TEMPORARY full-request debug dump on /v1/auth to
  learn how the speaker presents the app_key; to be removed later.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cmd/soundtouch-service/main.go

@@ -1283,6 +1283,11 @@ func setupRouter(server *handlers.Server, stockholmHandler *stockholm.Handler) *
 		r.Get("/blacklist/{deviceId}", func(w http.ResponseWriter, _ *http.Request) {
 			w.WriteHeader(http.StatusMethodNotAllowed)
 		})
+		// app_key validation for the /speaker notification endpoint. Real Bose
+		// validated the app_key against its cloud; as the cloud replacement we
+		// accept it (200). A 404 here makes the speaker report "invalid app key"
+		// (HandleInvalidAppKeyCb) and refuse TTS/URL notifications.
+		r.Get("/auth", server.HandleSpeakerAuth)
 	})
 
 	r.Route("/mgmt", func(r chi.Router) {

cmd/soundtouch-service/testdata/router_routes.txt

@@ -100,6 +100,7 @@ GET      /streaming/resources/api_versions.xml                        handlers.(
 GET      /streaming/software/update/account/{account}                 handlers.(*Server).HandleMargeSoftwareUpdate-fm
 GET      /streaming/sourceproviders                                   handlers.(*Server).HandleMargeSourceProviders-fm
 GET      /updates/soundtouch                                          handlers.(*Server).HandleMargeSoftwareUpdate-fm
+GET      /v1/auth                                                     handlers.(*Server).HandleSpeakerAuth-fm
 GET      /v1/blacklist/{deviceId}                                     setupRouter
 GET      /web/*                                                       setupRouter.(*Server).HandleWeb
 HEAD     /core02/svc-bmx-adapter-siriusxm-everest-eco1/prod/live-adapter handlers.(*Server).HandleSiriusXMLiveAdapter-fm