Add GitHub Actions CI workflow

[uwemaurer]

Sets up continuous integration via GitHub Actions.

What it does

• Runs on every push to master, every pull request to master, and manual dispatch.
• Uses Temurin JDK 21 and the project's Gradle wrapper (9.5.1).
• Runs ./gradlew build, which generates the OpenAPI sources, compiles, and runs the JUnit tests.
• Uploads test reports as a build artifact.

Verification

• ./gradlew clean build passes locally.
• This PR triggers the first CI run; merging after it goes green.

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