ci: use PACKAGE_TOKEN for the registry login

GITEA_TOKEN is not set in these repos — the registry login sent a blank
password and Gitea's /v2/ endpoint returned unauthorized. Switch the
image-build login, the app template, and the README to PACKAGE_TOKEN, the
PAT the other repos already use (Syrinx pairs gitea.actor + PACKAGE_TOKEN
successfully). The PAT needs write:package here and read:package in app
repos.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/builder-image.yml

@@ -38,7 +38,7 @@ jobs:
         with:
           registry: ${{ env.REGISTRY }}
           username: ${{ gitea.actor }}
-          password: ${{ secrets.GITEA_TOKEN }}
+          password: ${{ secrets.PACKAGE_TOKEN }}
 
       - name: Set up Buildx
         uses: docker/setup-buildx-action@v3