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>

config/prod.exs

@@ -19,5 +19,8 @@ config :logger, level: :info
 # Structured JSON logs for production (machine-parseable by fly logs, journalctl, Loki, etc.)
 config :logger, :default_handler, formatter: {LoggerJSON.Formatters.Basic, []}
 
+# Persistent image cache on the Fly volume (survives deploys)
+config :berrypod, :image_cache_dir, "/data/image_cache"
+
 # Runtime production configuration, including reading
 # of environment variables, is done on config/runtime.exs.