fix(tts): trimite lang=ro explicit la Supertonic API

Parametrul `lang` era definit (DEFAULT_LANG = "ro") dar nu era inclus
in request-ul HTTP catre /v1/audio/speech. Adaugat "lang": lang in
body-ul JSON si lang="ro" explicit in _tts_synthesize().

OpenAPI-ul Supertonic confirma ca /v1/audio/speech accepta `lang`
ca parametru optional (OpenAISpeechRequest schema).

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

tools/tts.py

@@ -46,6 +46,7 @@ def synthesize(text: str, voice: str = DEFAULT_VOICE, lang: str = DEFAULT_LANG)
                 "input": text,
                 "voice": voice,
                 "response_format": "wav",
+                "lang": lang,
             },
             timeout=60.0,
         )