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/controllers/checkout_controller.ex

@@ -1,7 +1,7 @@
 defmodule BerrypodWeb.CheckoutController do
   use BerrypodWeb, :controller
 
-  alias Berrypod.Cart
+  alias Berrypod.{Analytics, Cart}
   alias Berrypod.Orders
   alias Berrypod.Shipping
 
@@ -16,6 +16,7 @@ defmodule BerrypodWeb.CheckoutController do
       |> put_flash(:error, "Your basket is empty")
       |> redirect(to: ~p"/cart")
     else
+      track_checkout_start(conn)
       create_checkout(conn, hydrated)
     end
   end
@@ -126,4 +127,15 @@ defmodule BerrypodWeb.CheckoutController do
   end
 
   defp maybe_add_option(options, _result, _name, _min, _max), do: options
+
+  defp track_checkout_start(conn) do
+    visitor_hash = get_session(conn, "analytics_visitor_hash")
+
+    if visitor_hash do
+      Analytics.track_event("checkout_start", %{
+        pathname: "/checkout",
+        visitor_hash: visitor_hash
+      })
+    end
+  end
 end