Use GitHub Actions to ping the preview database

Hopefully this makes it so Aiven don't think it is not being used.

.github/workflows/cron.yml

@@ -0,0 +1,14 @@
+name: Cron
+
+on:
+  schedule:
+    - cron:  "0 6 * * 6" # 06:00 every Saturday
+  workflow_dispatch:
+
+jobs:
+  preview-psql:
+    runs-on: ubuntu-latest
+    env:
+      PREVIEW_DATABASE_URL: "${{ secrets.PREVIEW_DATABASE_URL }}"
+    steps:
+      - run: psql --command='SELECT version();' $PREVIEW_DATABASE_URL

README.md

@@ -111,6 +111,12 @@ fly secrets set --detach --app wikimum-preview DATABASE_URL="$(pbpaste)"
 fly secrets set --detach --app wikimum-preview SESSION_SECRET="$(pbpaste)"
 ```
 
+To keep the preview database (at Aiven) from being removed prematurely, GitHub Actions has been configured with the `cron.yml` workflow and this secret:
+
+```bash
+gh secret --repo Starkast/wikimum set PREVIEW_DATABASE_URL --body "$(pbpaste)"
+```
+
 ## Code Scanning
 
 GitHub Actions scan the code using [Brakeman](https://github.com/presidentbeef/brakeman).