v2.4.0

SiteGround invalidator: bulk flush when URL count exceeds threshold

Per-URL cache purging via sg_cachepress_purge_cache() opens a synchronous
Unix socket to SiteGround's Site Tools daemon for each URL. With large
tag sets (e.g. archive:mfn_news mapping to ~2000 URLs), this causes
thousands of blocking socket operations during PHP shutdown, holding
Apache worker threads and potentially crashing the server.

When URL count exceeds 50 (filterable via
cachetags/siteground-bulk-purge-threshold), perform a single full cache
flush instead.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>