add privacy-first analytics with progressive event collection

Three-layer pipeline: Plug for all HTTP requests (no JS needed), LiveView
hook for SPA navigations, JS hook for screen width. ETS-backed buffer
batches writes to SQLite every 10s. Daily-rotating salt for visitor hashing.
Includes admin dashboard with date ranges, visitor trends, top pages,
sources, devices, and e-commerce conversion funnel. Oban cron for 12-month
data retention.

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

lib/berrypod_web/router.ex

@@ -37,6 +37,7 @@ 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
@@ -63,7 +64,8 @@ defmodule BerrypodWeb.Router do
         {BerrypodWeb.ThemeHook, :mount_theme},
         {BerrypodWeb.ThemeHook, :require_site_live},
         {BerrypodWeb.CartHook, :mount_cart},
-        {BerrypodWeb.SearchHook, :mount_search}
+        {BerrypodWeb.SearchHook, :mount_search},
+        {BerrypodWeb.AnalyticsHook, :track}
       ] do
       live "/", Shop.Home, :index
       live "/about", Shop.Content, :about
@@ -170,6 +172,7 @@ defmodule BerrypodWeb.Router do
         {BerrypodWeb.AdminLayoutHook, :assign_current_path}
       ] do
       live "/", Admin.Dashboard, :index
+      live "/analytics", Admin.Analytics, :index
       live "/orders", Admin.Orders, :index
       live "/orders/:id", Admin.OrderShow, :show
       live "/products", Admin.Products, :index