Patch CVE-2026-66066 in Active Storage variant processing (#102)

Upgrade rails/activestorage 8.1.3 -> 8.1.3.1 (8-1-stable), which disables
libvips' unsafe "unfuzzed" format loaders/savers that allowed arbitrary file
read and RCE via crafted image uploads. Add a defense-in-depth initializer
calling Vips.block_untrusted(true) so untrusted loaders stay blocked even if
variant processing is introduced later.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: kcdragon