Fix Gitea Actions: drop custom job container, use setup-python

The python:3.11-slim job container lacked a Node.js runtime, so the
runner couldn't exec actions/checkout inside it. Run jobs on the
default runner image instead and pin Python via actions/setup-python.

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

deploy/systemd/README.md

@@ -58,5 +58,5 @@ usable there at all, fall back to a crontab entry instead:
 where you control a dedicated VM/server and need to serve the output yourself.
 On the ETP login nodes there's no Docker daemon (Apptainer/Singularity only),
 and `public_html` is already auto-served — running your own nginx there would
-be redundant. Docker is still used for CI (`.gitea/workflows/ci.yml`) and
-remains a fine option for anyone deploying this on their own server.
+be redundant. It remains a fine option for anyone deploying this on their own
+server.