Add a GitHub workflow cache to manually purge the Cloudflare cache (#2094)

## Changes

- Adds a workflow definition to manually trigger a Cloudflare purge

## Context

In #2086, and then also in
#2093, we saw problems where
the cache-purging logic that is supposed to be run as part of deploying
the site seems not to have worked as expected. The symptom is that the
CSS (whose file name now changes when its contents change) isn't
loading, because the front-page still serves an older version (one that
references a no-longer-existing CSS).

Let's add a way to manually purge the cache. It is totally possible that
this is one of those [famous "Close Door" elevator
buttons](https://www.sciencealert.com/the-close-door-buttons-in-elevators-don-t-actually-do-anything)
that fool us into believing that we did something, _anything_. But
_just_ in case it isn't such a no-op, let's have a GitHub workflow so
that we can trigger a Cloudflare cache purge manually.


[Here](https://github.com/git/git-scm.com/actions/runs/18134159656/job/51608395791)
is the workflow run that I would like to believe fixed the issue.

Author: dscho

.github/workflows/purge-cache.yml

@@ -0,0 +1,25 @@
+name: Purge Cloudflare Cache
+
+on:
+  workflow_dispatch:
+
+jobs:
+  purge-cache:
+    concurrency:
+      group: "purge-cache"
+      cancel-in-progress: false
+    runs-on: ubuntu-latest
+    steps:
+      - name: Purge Cloudflare cache
+        shell: bash
+        env:
+          CLOUDFLARE_TOKEN: ${{ secrets.CLOUDFLARE_TOKEN }}
+          CLOUDFLARE_ZONE: ${{ secrets.CLOUDFLARE_ZONE }}
+        if: env.CLOUDFLARE_TOKEN != ''
+        run: |
+          curl "https://api.cloudflare.com/client/v4/zones/$CLOUDFLARE_ZONE/purge_cache" \
+            -H "Authorization: Bearer $CLOUDFLARE_TOKEN" \
+            -H "Content-Type: application/json" \
+            -d '{ "purge_everything": true }'
+
+