ci: align workflows with pipeline-actions composite actions

- haven-notify.yaml: replace hand-rolled build+deploy (broken
  docker/build-push-action remote-builder path) with pipeline-actions
  build-and-push + deploy-restart; full Gitea uses: URLs; ubuntu-amd64.
- main.yaml / test-automated-nfs-backup.yaml: runner label
  ubuntu-latest -> ubuntu-amd64.
- haven-notify/README.md: point to infra/haven-notify.yaml in the
  haven repo; fix secret name to discord-webhook/HAVEN_WEBHOOK_URL.

.gitea/workflows/main.yaml

@@ -3,7 +3,7 @@ on: [push]
 
 jobs:
   check-scripts-syntax:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-amd64
     steps:
       - name: Check out repository code
         uses: actions/checkout@v4