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-alpine-323-arm64.yaml

@@ -57,3 +57,17 @@ steps:
       # options(future.globals.onReference = NULL): for some reason s3fs::file_delete() throws 'Error: Detected a non-exportable reference ('externalptr') in one of the globals ('FUN' of class 'function') used in the future expression' otherwise
       - xvfb-run /opt/R/$R_VERSION/bin/R -q -e "options(crayon.enabled = TRUE, Ncpus = 4, future.globals.onReference = 'error', repos = structure(c(getOption('repos'),INLA='https://inla.r-inla-download.org/R/stable'))); progressr::handlers('cli'); progressr::handlers(global = TRUE); options(future.globals.onReference = NULL); bincraft::process_cran_updates(interval = $INTERVAL, platform = 'alpine-323', process_updated = TRUE, process_new = TRUE, process_removed = TRUE, s3_endpoint = 'https://s3.eu-central-003.backblazeb2.com', s3_region = 'eu-central-003', s3_bucket = 'devxy-rpkgs-binaries', s3_access_key_id = Sys.getenv('B2_S3_ACCESS_KEY'), s3_secret_access_key = Sys.getenv('B2_S3_SECRET_KEY'), metadata_db_host = 'r-binaries.devxy.io', metadata_db_name = 'build_metadata', metadata_db_table = 'single_builds', metadata_db_user = 'rpkgs', metadata_db_password = Sys.getenv('PGPASS'), metadata_db_sslmode = 'require', metadata_db_port = 15432, archive = TRUE, upload = TRUE, store_build_metadata = TRUE)"
       - /opt/R/$R_VERSION/bin/R -q -e 'library(bincraft); upload_package_index(codename = "alpine323", s3_endpoint = "https://s3.eu-central-003.backblazeb2.com", s3_region = "eu-central-003", s3_bucket = "devxy-rpkgs-binaries", s3_access_key_id = Sys.getenv("B2_S3_ACCESS_KEY"), s3_secret_access_key = Sys.getenv("B2_S3_SECRET_KEY"))'
+
+  - 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: alpine323
+      ARCH: arm64
+    commands:
+      - apk add --no-cache -q bash curl
+      - bash scripts/purge_cdn_cache.sh "$BUNNYNET_API_KEY" "$ARCH" "$OS_ID" "$SUBDOMAIN1" "$SUBDOMAIN2" "$SUBDOMAIN3"