feat: implement Google OAuth authentication

- Add Google OAuth 2.0 login flow with passport-google-oauth20
- Create User and RefreshToken entities for session management
- Implement JWT access tokens (15min) + HttpOnly refresh cookies (7 days)
- Add auth endpoints: /google, /google/callback, /refresh, /me, /logout
- Create LoginPage with Google sign-in button (shadcn/ui)
- Add AuthGuard for protected routes with redirect preservation
- Implement silent token refresh on app mount
- Add UserMenu component with avatar and sign-out

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/app.module.ts

@@ -3,6 +3,7 @@ import { ConfigModule, ConfigService } from '@nestjs/config';
 import { TypeOrmModule } from '@nestjs/typeorm';
 import { ThumbnailsModule } from './modules/thumbnails/thumbnails.module';
 import { YouTubeModule } from './modules/youtube/youtube.module';
+import { AuthModule } from './modules/auth/auth.module';
 
 @Module({
   imports: [
@@ -25,6 +26,7 @@ import { YouTubeModule } from './modules/youtube/youtube.module';
     }),
     ThumbnailsModule,
     YouTubeModule,
+    AuthModule,
   ],
 })
 export class AppModule {}