Document and verify authored fork delta

[okturan]

What changed

• identifies this repository as a fork and links the upstream comparison for Okan's 10-commit delta
• documents the authored additions and fixes without claiming upstream work
• externalizes Compose credentials through an ignored local .env
• pins the application build and runtime to Java 17 and PostgreSQL 16
• adds least-privilege CI for the Maven verification, Compose contract, and production image build
• updates the PostgreSQL JDBC driver from 42.7.3 to 42.7.11

Why

The fork contains meaningful authored work, but its README did not clearly separate that work from the upstream project. Its containers also depended on floating base images and checked-in local credentials, and no workflow verified the contributed delta.

Impact

Recruiters and maintainers can now see exactly what the fork demonstrates. Local secrets stay outside Git, Java and PostgreSQL versions are deterministic, and every change is verified in CI against a live database.

Validation

• ./mvnw --batch-mode --no-transfer-progress verify in maven:3.9-eclipse-temurin-17 with PostgreSQL 16: passed
• Spring context test against PostgreSQL 16: passed
• docker-compose --env-file .env.example config --quiet: passed
• docker build --tag library-app-spring-boot:portfolio-check .: passed
• container smoke test: /v3/api-docs returned a 9,834-byte OpenAPI document
• portfolio tracked-credential scanner regression: passed
• git diff --check: passed