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/endpoint.ex

@@ -16,7 +16,7 @@ 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)
+  # In prod, image variants and mockups live on the persistent volume
   # rather than inside the ephemeral release directory.
   if image_cache_dir = Application.compile_env(:berrypod, :image_cache_dir) do
     plug Plug.Static,
@@ -25,6 +25,13 @@ defmodule BerrypodWeb.Endpoint do
       cache_control_for_etags: "public, max-age=31536000, immutable"
   end
 
+  if mockup_dir = Application.compile_env(:berrypod, :mockup_dir) do
+    plug Plug.Static,
+      at: "/mockups",
+      from: mockup_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.