feat: Add Interview Streak Tracking Feature (#625)

[Aasritha-sure]

Summary

This PR introduces an Interview Streak Tracking feature to encourage users to practice consistently.

Changes Made

• Added interview streak tracking.
• Displays current streak and longest streak.
• Resets streak after missed practice days.
• Added milestone badges for 3, 7, and 30-day streaks.
• Prevented duplicate streak increments on the same day.
• Integrated streak information into the dashboard.
• Added/updated tests covering streak calculations and badge unlocking.
• Kept the implementation consistent with the existing project architecture.

Fixes #625

Looks good to me. Ready to merge.

[github-actions]

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[coderabbitai]

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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 945ed5e and 4172f3a.

⛔ Files ignored due to path filters (1)

• backend/package-lock.json is excluded by !**/package-lock.json

📒 Files selected for processing (14)

• backend/.env.example
• backend/config/validateEnv.js
• backend/controllers/googleCalendarController.js
• backend/controllers/sessionController.js
• backend/models/User.js
• backend/package.json
• backend/routes/googleCalendarRoutes.js
• backend/server.js
• backend/services/googleCalendarService.js
• backend/tests/googleCalendarController.unit.test.js
• backend/tests/interviewStreak.unit.test.js
• backend/utils/interviewStreak.js
• frontend/src/pages/Home/CreateSessionForm.jsx
• frontend/src/pages/Home/Dashboard.jsx

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📝 Walkthrough

Walkthrough

The PR adds interview streak tracking across session creation and the dashboard. It also adds optional Google Calendar OAuth, encrypted token storage, protected API routes, connection status reporting, and event synchronization.

Changes

Interview streak tracking

Layer / File(s)	Summary
Streak calculation and validation backend/utils/interviewStreak.js, backend/tests/interviewStreak.unit.test.js	Calculates consecutive and longest streaks, handles duplicate or invalid dates, and awards milestone badges.
Session persistence and response backend/controllers/sessionController.js	Updates streak fields during session creation and returns a streakSummary. Missing users return HTTP 404.
Dashboard streak display frontend/src/pages/Home/CreateSessionForm.jsx, frontend/src/pages/Home/Dashboard.jsx	Updates shared user state after session creation and displays streak totals and badges.

Google Calendar integration

Layer / File(s)	Summary
OAuth configuration and credential storage backend/.env.example, backend/config/validateEnv.js, backend/package.json, backend/models/User.js, backend/services/googleCalendarService.js	Adds optional OAuth configuration, Google API support, Calendar metadata fields, and AES-256-GCM token encryption.
OAuth and connection endpoints backend/controllers/googleCalendarController.js, backend/routes/googleCalendarRoutes.js, backend/server.js, backend/tests/googleCalendarController.unit.test.js	Exposes connection, callback, status, and event-sync endpoints with authentication protection except for the OAuth callback.
Calendar event synchronization backend/services/googleCalendarService.js, backend/controllers/googleCalendarController.js	Builds UTC event payloads, refreshes expired tokens, creates or updates events, records sync time, and returns validation or expiration errors.

Estimated code review effort: 4 (Complex) | ~45 minutes

Sequence Diagram(s)

sequenceDiagram
  participant Client
  participant googleCalendarRoutes
  participant googleCalendarController
  participant googleCalendarService
  participant GoogleOAuth
  participant User
  Client->>googleCalendarRoutes: Request Calendar connection
  googleCalendarRoutes->>googleCalendarController: Call connectGoogleCalendar
  googleCalendarController->>googleCalendarService: Generate authorization URL
  googleCalendarService-->>Client: Return authorization URL
  Client->>GoogleOAuth: Approve Calendar access
  GoogleOAuth->>googleCalendarRoutes: Redirect with code and state
  googleCalendarRoutes->>googleCalendarController: Call handleGoogleCalendarCallback
  googleCalendarController->>googleCalendarService: Exchange authorization code
  googleCalendarService->>GoogleOAuth: Return OAuth tokens
  googleCalendarController->>User: Store encrypted tokens and metadata

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Possibly related PRs

• Canopus-Labs/PrepPilot#1369: Implements related interview-preparation streak and badge concepts in different components.
• Canopus-Labs/PrepPilot#1511: Adds frontend integration supported by this PR’s Google Calendar backend.

Suggested reviewers: karanunique

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[KaranUnique]

@Aasritha-sure Please take look github actions suggestions, once it is addressed, I'll merge this PR