feat : added Google Calendar backend OAuth2 routes

[tmdeveloper007]

Summary of What Has Been Done

Created backend/routes/googleCalendarRoutes.js with four routes that the frontend Interview Prep page already calls via API_PATHS.GOOGLE_CALENDAR:

• GET /connect — initiates Google OAuth2 flow and returns { authUrl }
• GET /callback — handles the OAuth2 redirect, exchanges code for token
• GET /status — returns { connected, email } for the authenticated user (stub)
• POST /events — creates a calendar event for the interview session (stub)

Mounted the router at /api/google-calendar in backend/server.js.

Changes Made

• backend/routes/googleCalendarRoutes.js (new): all four routes with input validation and stub responses
• backend/server.js: added import and app.use("/api/google-calendar", ...) mount

Impact it Made

The Google Calendar connect/sync UI in the Interview Prep page no longer returns 404. The /connect and /callback routes provide a foundation for real OAuth2 integration when GOOGLE_CLIENT_ID and GOOGLE_CLIENT_SECRET env vars are set.

Closes #1793
Closes #1927

Note: Please assign this PR to the tmdeveloper007 account.

Summary

Adds Google Calendar API routes and mounts them at /api/google-calendar.

• Adds OAuth connect and callback endpoints.
• Adds authenticated status and event creation endpoints.
• Validates OAuth parameters, event fields, and ISO dates.
• Returns 503 when OAuth credentials are not configured.
• Provides a foundation for future Google Calendar integration.

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ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 206d384 and e6e967d.

📒 Files selected for processing (2)

• backend/routes/googleCalendarRoutes.js
• backend/server.js

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📝 Walkthrough

Walkthrough

The backend adds Google Calendar OAuth, status, and event endpoints. It validates OAuth and event input, handles token exchange errors, redirects successful callbacks, and mounts the router under /api/google-calendar.

Changes

Google Calendar backend integration

Layer / File(s)	Summary
Route registration and OAuth initiation backend/routes/googleCalendarRoutes.js, backend/server.js	The router reads Google OAuth settings, builds the authorization URL, exports the router, and mounts it with generalLimiter.
OAuth callback exchange backend/routes/googleCalendarRoutes.js	The callback validates parameters, exchanges the authorization code, handles provider and network errors, and redirects on success.
Calendar status and event endpoints backend/routes/googleCalendarRoutes.js	Authenticated endpoints report disconnected status and validate event titles and ISO date values before returning stub responses.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Sequence Diagram(s)

sequenceDiagram
  participant InterviewPrep
  participant googleCalendarRoutes
  participant GoogleOAuth
  participant Frontend
  InterviewPrep->>googleCalendarRoutes: GET /api/google-calendar/connect
  googleCalendarRoutes->>GoogleOAuth: Request authorization URL
  GoogleOAuth-->>googleCalendarRoutes: OAuth callback with code
  googleCalendarRoutes->>GoogleOAuth: Exchange code for tokens
  googleCalendarRoutes-->>Frontend: Redirect after callback

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Possibly related PRs

• Canopus-Labs/PrepPilot#1511: Adds the frontend integration consumed by the Google Calendar backend routes.
• Canopus-Labs/PrepPilot#1513: Directly modifies the Google Calendar routes and server wiring.
• Canopus-Labs/PrepPilot#1804: Implements a stubbed version of the Google Calendar OAuth and synchronization endpoints.

Suggested labels: type:feature

Suggested reviewers: karanunique

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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