chore: add automated user-service TypeScript types generation script

[aminamos]

Summary:

Resolves #154.
Adds scripts/generate-user-api-types.mjs and updates package.json with generate:user-api-types to generate typed API definitions directly from OpenAPI artifacts.

Expected behavior:

Running yarn generate:user-api-types generates up-to-date user-service-api-types.ts.

Testing tips:

1. Checkout feat/154-user-service-types-artifact.
2. Run node scripts/generate-user-api-types.mjs.

Please make sure these boxes are checked before submitting your pull request - thanks!

• Run the unit tests with yarn test to make sure you didn't break anything
• Add or update any needed documentation to the repo
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
• Include screenshot(s) showing how this pull request works and fixes the issue(s)

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