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>

config/prod.exs

@@ -19,8 +19,9 @@ 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)
+# Persistent image cache and mockup variants on the Fly volume (survives deploys)
 config :berrypod, :image_cache_dir, "/data/image_cache"
+config :berrypod, :mockup_dir, "/data/mockups"
 
 # Runtime production configuration, including reading
 # of environment variables, is done on config/runtime.exs.