feat(tts): default to /speaker playback; drop /v1/auth debug dump

Confirmed working on a real speaker (Bose_Lisa/27.0.6): the speaker GETs
/v1/auth at audionotification.api.bosecm.com (DNS-redirected to us) with
the app_key in an "Apikeyheader" header, and an empty 200 is sufficient.

- Make "speaker" the default playback method (ducks + resumes the current
  playback, supports volume) for the speak endpoint, the CLI --method flag,
  and the web UI button; "radio" remains opt-in.
- Remove the temporary full-request debug dump from /v1/auth now that the
  contract is understood; document it in the handler comment instead.

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

cmd/soundtouch-cli/cmd_tts.go

@@ -54,8 +54,8 @@ func ttsCloudCmd() *cli.Command {
 			},
 			&cli.StringFlag{
 				Name:  "method",
-				Usage: "Playback method: 'radio' (LOCAL_INTERNET_RADIO, no app_key, replaces source) or 'speaker' (/speaker notification, ducks+resumes, supports volume)",
-				Value: "radio",
+				Usage: "Playback method: 'speaker' (/speaker notification, ducks+resumes, supports volume) or 'radio' (LOCAL_INTERNET_RADIO, no app_key, replaces source)",
+				Value: "speaker",
 			},
 		),
 		Action: ttsCloud,