feat: implement OAuth2 Google and Apple authentication (#142)

[spiffamani]

Closes #142

Summary

Implemented OAuth2 authentication flows for Google and Apple Sign In with user profile mapping and account linking.

New Files

backend/src/auth/entities/oauth-user.entity.ts

• New entity to store OAuth provider links per user
• Supports google and apple providers via enum
• Stores provider user ID, access token, and refresh token

backend/src/auth/dto/oauth.dto.ts

• GoogleOAuthDto — validates Google OAuth profile data
• AppleOAuthDto — validates Apple Sign In profile data
• OAuthCallbackDto — validates OAuth callback code and state

backend/src/auth/services/oauth.service.ts

• Handles user profile mapping on first OAuth login
• Automatically creates new user account if email not found
• Links OAuth account to existing user if email already exists
• Generates JWT access and refresh tokens on successful OAuth login

backend/src/auth/strategies/google.strategy.ts

• Passport Google OAuth2 strategy using passport-google-oauth20
• Scopes: email, profile
• Reads credentials from environment variables

backend/src/auth/strategies/apple.strategy.ts

• Passport Apple Sign In strategy using passport-apple
• Validates Apple ID token and maps profile data
• Reads credentials from environment variables

Updated Files

backend/src/auth/auth.module.ts

• Registered GoogleStrategy, AppleStrategy, OAuthService
• Added OAuthUser entity to TypeORM feature list

backend/src/auth/auth.controller.ts

• Added GET /auth/google — initiates Google OAuth2 flow
• Added GET /auth/google/callback — handles Google callback
• Added GET /auth/apple — initiates Apple Sign In flow
• Added POST /auth/apple/callback — handles Apple callback

Environment Variables Required

GOOGLE_CLIENT_ID=
GOOGLE_CLIENT_SECRET=
GOOGLE_CALLBACK_URL=http://localhost:3000/auth/google/callback
APPLE_CLIENT_ID=
APPLE_TEAM_ID=
APPLE_KEY_ID=
APPLE_PRIVATE_KEY=
APPLE_CALLBACK_URL=http://localhost:3000/auth/apple/callback

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