Fix/google calendar backend routes

[Suprita736]

📝 Pull Request Description

Related Issue

Closes #1793

Summary

Implement the missing backend Google Calendar API required by the existing Interview Prep frontend.

This fixes the Google Calendar functionality that was previously failing because the frontend called /api/google-calendar/* routes that were not mounted by the backend.

The implementation adds the required backend routes for:

• Starting the Google OAuth flow
• Handling the OAuth callback
• Checking the user's Google Calendar connection status
• Syncing Interview Prep events to Google Calendar

Google Calendar credentials are handled on the backend and scoped to the authenticated user.

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Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature
• ♻️ Refactoring
• 📝 Documentation update
• 🎨 UI/UX improvement
• 🔥 Other(please describe) ______

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How Has This Been Tested?

• Added unit tests covering the Google Calendar controller behavior.
• Tested authenticated Google Calendar status/connect/sync behavior.
• Tested OAuth callback and error handling.
• Verified Google Calendar credentials remain user-scoped and are not exposed in API responses.
• Ran the relevant backend tests/validation checks.

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Screenshots (if applicable)

Not applicable — this is a backend/API fix.

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Checklist

• My code follows the project's guidelines
• I have tested my changes
• I have updated documentation where necessary
• I have linked the related issue
• My changes do not introduce new warnings or errors

[github-actions]

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📥 Commits

Reviewing files that changed from the base of the PR and between 420348a and fbc3157.

📒 Files selected for processing (6)

• backend/controllers/googleCalendarController.js
• backend/models/User.js
• backend/routes/googleCalendarRoutes.js
• backend/server.js
• backend/tests/googleCalendarController.unit.test.js
• backend/tests/interviewExperienceController.unit.test.js

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