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/components/layouts/admin.html.heex

@@ -62,6 +62,14 @@
               <.icon name="hero-home" class="size-5" /> Dashboard
             </.link>
           </li>
+          <li>
+            <.link
+              navigate={~p"/admin/analytics"}
+              class={admin_nav_active?(@current_path, "/admin/analytics")}
+            >
+              <.icon name="hero-chart-bar" class="size-5" /> Analytics
+            </.link>
+          </li>
           <li>
             <.link
               navigate={~p"/admin/orders"}