fix: Generate the OAuth 2.0 keys at install

Problem/Motivation

POST /oauth/token answers 500: "You need to set the OAuth2 private key." Simple OAuth 6.1.4 is installed by the recipe and no key pair is generated or configured; /oauth/jwks is {"keys":[]}. The README promises OAuth 2.0.

Steps to reproduce

1. Build a site from a recipe that applies modules/contrib/webapi/recipes/default.
2. Create a consumer with the client_credentials grant.
3. POST /oauth/token with the client id, secret and a scope: 500.

Proposed resolution

Generate the key pair in private://simple_oauth at install and in an update hook when the site has a private files directory and no keys are configured, and warn on the status report while no readable key pair exists.

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ✅ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ❌ Update Release Notes
• ❌ Release

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• fix: Generate the OAuth 2.0 keys at install

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