add analytics CSV export

Downloads a ZIP with one CSV per report (overview, trend, pages, entry/exit
pages, sources, referrers, countries, devices, funnel). Export button lives
next to the period selector and picks up the current period and any active
filters using a JS hook + JS.set_attribute, so the downloaded data always
matches what's on screen.

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

lib/berrypod_web/router.ex

@@ -187,6 +187,8 @@ defmodule BerrypodWeb.Router do
   scope "/admin", BerrypodWeb do
     pipe_through [:browser, :require_authenticated_user, :admin]
 
+    get "/analytics/export", AnalyticsExportController, :export
+
     live_session :admin,
       layout: {BerrypodWeb.Layouts, :admin},
       on_mount: [