Harden tests, CI and project documentation

[DataTideHH]

What changed

• adds a Java 21 GitHub Actions workflow using the Maven Wrapper and clean verify
• expands API integration coverage for POST, validation, PUT, DELETE and missing-resource behavior
• documents the status filter, HTTP success codes and Problem Detail error responses
• adds Windows PowerShell Maven Wrapper commands alongside macOS/Linux commands
• completes Maven project metadata for the public repository
• updates the GitHub Pages project site to match the tested API contract
• links the official Spring Boot, Spring Data JPA, Maven Wrapper and GitHub Actions references curated in open-learning-resources

Scope

No production endpoints, runtime dependencies or persistence technology were added. The PR only verifies and documents the API behavior already implemented by the repository.

Validation target

The new Java 21 workflow runs:

./mvnw --batch-mode --no-transfer-progress clean verify

The integration suite now covers:

• list and status-filter behavior
• invalid status handling
• lookup and 404 Problem Details
• successful creation with 201 and Location
• request validation failures
• persisted updates
• successful deletion with 204
• update and delete attempts for unknown IDs