feat(tts): upgrade to pcm_24000 encoding + language=de

Switch from mp3_22050_32 (default) to lossless PCM 24kHz for cleaner
voice output. Add language=de for German text normalization.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

voice.py

@@ -591,8 +591,14 @@ class VoiceSession:
                     http_session=self._http_session,
                 ),
                 llm=lk_openai.LLM(base_url=LITELLM_URL, api_key=LITELLM_KEY, model=self.model),
-                tts=elevenlabs.TTS(voice_id=voice_id, model="eleven_multilingual_v2",
-                                   api_key=ELEVENLABS_KEY, http_session=self._http_session),
+                tts=elevenlabs.TTS(
+                    voice_id=voice_id,
+                    model="eleven_multilingual_v2",
+                    encoding="pcm_24000",
+                    language="de",
+                    api_key=ELEVENLABS_KEY,
+                    http_session=self._http_session,
+                ),
                 vad=_get_vad(),
             )