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/shop.html.heex

@@ -1,2 +1,3 @@
 <.shop_flash_group flash={@flash} />
+<div id="analytics-init" phx-hook="AnalyticsInit" phx-update="ignore" style="display:none"></div>
 {@inner_content}