feat(auth): implement Google and Zalo OAuth backend strategies

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>

.env.example

@@ -60,6 +60,19 @@ JWT_EXPIRES_IN=15m
 JWT_REFRESH_SECRET=CHANGE_ME
 JWT_REFRESH_EXPIRES_IN=7d
 
+# -----------------------------------------------------------------------------
+# OAuth Providers
+# -----------------------------------------------------------------------------
+GOOGLE_CLIENT_ID=
+GOOGLE_CLIENT_SECRET=
+GOOGLE_CALLBACK_URL=http://localhost:3001/auth/google/callback
+
+ZALO_APP_ID=
+ZALO_APP_SECRET=
+ZALO_CALLBACK_URL=http://localhost:3001/auth/zalo/callback
+
+FRONTEND_URL=http://localhost:3000
+
 # -----------------------------------------------------------------------------
 # Next.js Web
 # -----------------------------------------------------------------------------