Derive OAuth callback URL and prefix stack resources

[ltyu]

Summary

• derive the Google OAuth callback URL from the deployed HttpApi request context and request the identity scopes needed for the app-authenticated flow
• parameterize named AWS resources and expose OAuth stack outputs so staging-style deployments do not require hard-coded callback settings
• align docs and package install behavior by removing the callback env var dependency and making prepare tolerant when husky is unavailable

Testing

• npm run test:run -- src/handlers/startGoogleOAuth.test.ts
• npx tsc --noEmit
• npm run build