fix(ci): use REGISTRY secrets for container registry login

GITHUB_TOKEN doesn't have container registry permissions in Gitea.
Use dedicated REGISTRY_USERNAME and REGISTRY_PASSWORD secrets.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.gitea/workflows/release.yml

@@ -30,8 +30,8 @@ jobs:
         uses: docker/login-action@v3
         with:
           registry: tea.millaguie.net
-          username: ${{ github.actor }}
-          password: ${{ secrets.GITHUB_TOKEN }}
+          username: ${{ secrets.REGISTRY_USERNAME }}
+          password: ${{ secrets.REGISTRY_PASSWORD }}
 
       - name: Build and push backend image
         uses: docker/build-push-action@v5