persist image cache on fly volume across deploys

Image variants were written to the ephemeral release directory
and wiped on every deploy, causing 500 errors with 50s timeouts
as browsers waited for images that could never be served.

- Point image_cache_dir at /data/image_cache in prod
- Add Plug.Static to serve from the persistent volume
- Exclude /image_cache/ from broken URL tracking

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

lib/berrypod_web/plugs/broken_url_tracker.ex

@@ -34,6 +34,7 @@ defmodule BerrypodWeb.Plugs.BrokenUrlTracker do
   defp static_path?(path) do
     String.starts_with?(path, "/assets/") or
       String.starts_with?(path, "/images/") or
+      String.starts_with?(path, "/image_cache/") or
       String.starts_with?(path, "/favicon")
   end
 end