feat: add withRetry utility and wrap all transcription services

[Utkarsh9571]

This PR introduces a reusable withRetry utility for retrying async operations with delay and timeout. It wraps all transcription services (openai.ts, groq.ts, deepgram.ts, elevenlabs.ts, mistral.ts) with contributor-safe retry logic.

Retries: 2

Delay: 1000ms

Timeout: 8000ms

Also added documentation to services/README.md. No tests added yet — happy to follow up if needed.