Add complete OAuth2 authentication flow for Google and Zalo providers:
- OAuthService: handles account linking (by email/phone), new user
creation for OAuth-only accounts, and JWT token generation
- GoogleOAuthStrategy: passport-google-oauth20 integration
- ZaloOAuthStrategy: custom OAuth2 implementation using Zalo's API
(authorization URL generation, code exchange, user info fetch)
- OAuthController: redirect and callback endpoints for both providers
with httpOnly cookie-based token management
- Unit tests for OAuthService (7 tests), GoogleOAuthStrategy (4 tests),
and ZaloOAuthStrategy (7 tests)
- OAuth env vars added to .env.example and env-validation warnings
Co-Authored-By: Paperclip <noreply@paperclip.ing>
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