feat: establish Backstage overlay contract and verification

[DerekRoberts]

Establish and document the Backstage software template overlay contract for swapping backend modules into quickstart-openshift, complete with consumer integration steps and compatibility verification.

• Adds OVERLAY.md specifying canonical Backstage software template actions (fetch:plain, fs:delete), file mapping contracts, OpenShift template parameters, and overlay verification checklists.
• Documents the standardized docker-compose.yml backend swap service for live-reload Java 21 dev mode in stamped repositories.
• Reverts Renovate's auto-upgrade of eclipse-temurin:27-jdk back to 21-jdk and adds a packageRule in renovate.json locking it to Java 21 LTS (<22).
• Updates backend-java/README.md prerequisites to Java 21 JDK.

Closes #488

• Java
• Py

[Copilot]

Copilot review overview

🟡 Changes recommended

The canonical Backstage example is invalid and the documented verification does not exercise the Compose overlay.

Review effort: Balanced
Findings: 2 · 2

Open (4)

• Use the correct Backstage Scaffolder template API version · New
• Request a valid GitHub repository owner, not a team or group · New
• Add the missing backend-py .dockerignore or narrow the requirement · New
• Apply the compose backend replacement before compatibility verification · New

What changed in this PR

Establishes the Backstage backend-overlay contract and standardizes Java 21 development.

Changes:

• Documents Backstage overlay, deployment, Compose, and verification workflows.
• Pins Eclipse Temurin development images below Java 22.
• Updates Java prerequisites to JDK 21.

File	Description
OVERLAY.md	Defines the overlay contract and workflow.
README.md	Links to the overlay documentation.
docker-compose.yml	Uses Eclipse Temurin 21.
backend-java/​README.md	Updates the JDK prerequisite.
renovate.json	Prevents Temurin upgrades beyond Java 21.

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