feat(ci): purge CDN cache after process-updates runs (#71)

## Summary

- `process_cran_updates()` ends with `upload_package_index()`, refreshing `src/contrib/PACKAGES{,.gz,.rds,.db}` in S3 — but no CDN purge follows.
- The CDN was only purged by the daily `update-package-index-*` chain, so between cron runs the edge kept serving stale indexes. A binary that landed in S3 was effectively invisible to `install.packages()` until the index-update chain caught up.

Adds the same `Purge CDN cache` step that `update-package-index-*` already uses, parameterized per platform/arch. The script and image are unchanged.

Reviewed-on: #71

.crow/process-updates-redhat-10-amd64.yaml

@@ -77,3 +77,17 @@ steps:
             operator: 'Equal'
             value: 'true'
             effect: 'NoSchedule'
+
+  - name: Purge CDN cache
+    image: reg.devxy.io/docker.io/library/alpine:3.23
+    environment:
+      BUNNYNET_API_KEY:
+        from_secret: BUNNYNET_API_KEY
+      SUBDOMAIN1: 'cran.devxy.io'
+      SUBDOMAIN2: 'cran.allianceswisspass.devxy.io'
+      SUBDOMAIN3: 'cran.rpkgs.com'
+      OS_ID: rhel10
+      ARCH: amd64
+    commands:
+      - apk add --no-cache -q bash curl
+      - bash scripts/purge_cdn_cache.sh "$BUNNYNET_API_KEY" "$ARCH" "$OS_ID" "$SUBDOMAIN1" "$SUBDOMAIN2" "$SUBDOMAIN3"