persist mockup variants on fly volume across deploys

Source mockup WebPs are copied from the release to /data/mockups/
on startup, and variants are generated there. This eliminates the
182-job storm on every deploy that was saturating the CPU and
causing SQLite locking. After the first successful run, subsequent
deploys find all variants intact.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/berrypod_web/plugs/broken_url_tracker.ex

@@ -35,6 +35,7 @@ defmodule BerrypodWeb.Plugs.BrokenUrlTracker do
     String.starts_with?(path, "/assets/") or
       String.starts_with?(path, "/images/") or
       String.starts_with?(path, "/image_cache/") or
+      String.starts_with?(path, "/mockups/") or
       String.starts_with?(path, "/favicon")
   end
 end