cache settings, categories, and media in ETS to cut per-request DB queries

Every shop page load was triggering ~18 DB queries for data that rarely
changes (theme settings, nav items, categories, shipping countries, logo,
header image). On a shared-cpu-1x Fly machine with SQLite this was the
primary performance bottleneck.

- Add SettingsCache GenServer+ETS for all non-encrypted settings
- Cache list_categories() with single-query N+1 fix (correlated subquery)
- Cache list_available_countries_with_names() in shipping
- Cache Media.get_logo() and Media.get_header()
- Remove duplicate LoadTheme plug from :shop and :admin pipelines
- Invalidate caches on writes (put_setting, product sync, media upload)
- Clear caches between tests via DataCase/ConnCase setup

Per-page queries reduced from ~18 to ~2.

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

lib/berrypod_web/router.ex

@@ -42,13 +42,11 @@ defmodule BerrypodWeb.Router do
 
   pipeline :shop do
     plug :put_root_layout, html: {BerrypodWeb.Layouts, :shop_root}
-    plug BerrypodWeb.Plugs.LoadTheme
     plug BerrypodWeb.Plugs.Analytics
   end
 
   pipeline :admin do
     plug :put_root_layout, html: {BerrypodWeb.Layouts, :admin_root}
-    plug BerrypodWeb.Plugs.LoadTheme
   end
 
   # ── Routes without the :browser pipeline ──────────────────────────