Flatten admin image name to andras/fizika-admin

A nested name (andras/fizika/fizika-admin) contains a slash, which Forgejo's
package web UI can't render (the detail page 404s though pulls work). Use a
flat name to match the andras/<image> convention.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.forgejo/workflows/docker-publish.yml

@@ -20,6 +20,6 @@ jobs:
         uses: http://forgejo:3000/andras/ci-actions/docker-publish@main
         with:
           context: ./backend
-          image-suffix: /fizika-admin
+          image-suffix: -admin
           push: ${{ github.event_name != 'pull_request' }}
           token: ${{ secrets.FORGEJO_PACKAGE_TOKEN }}