Added 60dB integration

[manishEMS47]

What we added
Added 60db as a third text-to-speech provider alongside OpenAI and ElevenLabs:

• sixtydb_text_to_speech() — streams audio from 60db's /tts-stream endpoint (NDJSON, base64 chunks), saves to outputs/, and plays it.
• play_audio() — cross-platform playback (Windows/macOS/Linux); also fixed the OpenAI path which was macOS-only before.
• "60db" added to the -tts flag + config env vars (SIXTYDB_API_KEY, SIXTYDB_VOICE_ID).
• Updated requirements.txt (requests, python-dotenv) and README.md.

How to use

1. Install deps: pip install -r requirements.txt
2. Add to .env:
   OPENAI_API_KEY=sk-... # always needed (speech-to-text + chat)
   SIXTYDB_API_KEY=sk_live_...
   SIXTYDB_VOICE_ID= # optional; omit for default voice
3. Run with 60db voice:
   python main.py -pf prompts/vet_prompt.md -tts 60db

When it says "Listening...", speak — ChatGPT replies in the 60db voice. Say "bye" to end the call.