Add API Key Authentication to All AI4Bharat Services

[shubhh139]

Implement API key authentication for TTS, STT, and Transliteration endpoints to match the existing translation endpoint security model. This will ensure all AI4Bharat services require valid API keys for access.

Acceptance Criteria

• Add require_api_key dependency to TTS endpoint (/api/v1/tts)
• Add require_api_key dependency to STT endpoint (/api/v1/stt)
• Add require_api_key dependency to transliteration endpoint (/api/v1/transliterate)
• Ensure all endpoints return 401 when no API key is provided
• Ensure all endpoints return 401 when invalid API key is provided
• Update tests to verify API key protection on all endpoints
• Update Postman collections with API key headers
• Update documentation with API key requirements

Estimation: 4 hours