fix: add sample environment variables and update README for Okta integration

[lcaohoanq]

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Description

This PR updates the documentation and configuration to improve local development experience.

• README: Fixed the example configuration line for redirect_uri so that it correctly matches the required callback URL.

• .gitignore: Added .env and .env.sample to avoid committing sensitive environment variables.

• .env.sample: Introduced a sample file to serve as a reference for retrieving environment variables in application.yml.

These changes provide a clearer onboarding experience for new developers and prevent accidental leaks of sensitive environment data.
No breaking changes introduced.

Testing

Steps performed:

• Pulled the updated branch.

• Verified that redirect_uri in README.md matches the allowed callback URL configured in the Auth0 dashboard.

• Confirmed .env is ignored by Git and .env.sample is included.

• Started the app locally (localhost:3000) with Auth0 login flow and confirmed successful login without "Callback URL mismatch" error.

Environment:

• Spring Boot 3.x with OAuth2 client

• Java 17

• Auth0 tenant: dev-pwuxxxxxxxxxxx2e.us.auth0.com

No new unit tests were required since changes are configuration/docs only.

• This change does not affect existing test coverage.

Checklist

• I have added documentation for new/changed functionality in this PR or in auth0.com/docs
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used, if not master