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/endpoint.ex

@@ -16,6 +16,15 @@ defmodule BerrypodWeb.Endpoint do
     websocket: [connect_info: [session: @session_options]],
     longpoll: [connect_info: [session: @session_options]]
 
+  # In prod, image variants live on the persistent volume (/data/image_cache)
+  # rather than inside the ephemeral release directory.
+  if image_cache_dir = Application.compile_env(:berrypod, :image_cache_dir) do
+    plug Plug.Static,
+      at: "/image_cache",
+      from: image_cache_dir,
+      cache_control_for_etags: "public, max-age=31536000, immutable"
+  end
+
   # Serve at "/" the static files from "priv/static" directory.
   # gzip only in prod — avoids stale .gz files from mix assets.deploy
   # shadowing freshly-built dev assets.