Fix #166: Implement Gemini API request timeout handling

[Aasritha-sure]

Description

This PR implements configurable timeout handling for Gemini API requests using AbortController.

Changes Made

• Added configurable GEMINI_TIMEOUT environment variable (default: 30000 ms).
• Implemented timeout handling for Gemini API generation requests.
• Added request cancellation using AbortController.
• Added timeout logging for monitoring and debugging.
• Added meaningful timeout responses with HTTP 504 status codes.
• Preserved existing functionality when requests complete successfully.

Files Modified

• backend/.env.example
• backend/controllers/aiController.js
• backend/controllers/resumeController.js
• backend/routes/AptitudeQuestions.js
• backend/routes/aiRoutes.js

Testing

• Verified timeout configuration is loaded from environment variables.
• Verified timeout errors return appropriate responses.
• Confirmed existing Gemini request flow remains unchanged for successful requests.

Fixes #166