⚡ Bolt: Use AsyncGroq to prevent blocking the event loop

[Adityasingh-8858]

💡 What: Replaced the synchronous Groq client with AsyncGroq in backend/main.py and updated ai_voice and initiate_transfer to await the completions.
🎯 Why: Synchronous network calls (like the standard Groq client) block the FastAPI asynchronous event loop, creating a significant concurrency bottleneck.
📊 Impact: Prevents thread blocking during Groq LLM API requests, significantly improving the server's ability to handle concurrent requests efficiently.
🔬 Measurement: Verify by executing parallel requests to the endpoints and observing reduced latency / unblocked response times compared to the sync client. Tests were run and passed.

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PR created automatically by Jules for task 2905773569666048901 started by @Deepaksingh7238

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