CI: PR workflow (mvn verify)

[Robert27]

Goal

Run mvn verify on every pull request to main.

Tasks

• GitHub Actions workflow on pull_request targeting main
• JDK 25 (Temurin) with Maven dependency cache
• Run mvn -B verify (compile, tests, format checks once configured)

Acceptance criteria

• Workflow runs on every PR
• Failed builds block merge via branch protection status check