Use dedicated PAT for registry login in Gitea Actions workflow

The built-in actions token lacks package registry write scope,
causing docker login to fail with "unauthorized". A PAT with
write:package scope must be added as the REGISTRY_TOKEN secret.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -23,7 +23,7 @@ jobs:
         with:
           registry: ${{ env.REGISTRY }}
           username: ${{ gitea.actor }}
-          password: ${{ secrets.GITHUB_TOKEN }}
+          password: ${{ secrets.REGISTRY_TOKEN }}
 
       - name: Build and push Backend
         uses: docker/build-push-action@v5