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/plugs/analytics.ex

@@ -0,0 +1,112 @@
+defmodule BerrypodWeb.Plugs.Analytics do
+  @moduledoc """
+  Plug that records a pageview event on every shop HTTP request.
+
+  This is Layer 1 of the progressive analytics pipeline — it fires on every
+  GET request regardless of whether JS is enabled. Computes a privacy-friendly
+  visitor hash, parses the user agent, extracts referrer and UTM params, and
+  buffers the event for batch writing to SQLite.
+
+  Also stores analytics data in the session so the LiveView hook (Layer 2)
+  can read it for subsequent SPA navigations.
+  """
+
+  import Plug.Conn
+
+  alias Berrypod.Analytics
+  alias Berrypod.Analytics.{Salt, UAParser, Referrer}
+
+  def init(opts), do: opts
+
+  def call(%{method: "GET"} = conn, _opts) do
+    ua = get_req_header(conn, "user-agent") |> List.first("")
+    {browser, os} = UAParser.parse(ua)
+
+    # Skip bots
+    if browser == "Bot" do
+      conn
+    else
+      # Skip if the logged-in admin is browsing
+      if admin?(conn) do
+        prepare_session(conn, ua, browser, os)
+      else
+        record_and_prepare(conn, ua, browser, os)
+      end
+    end
+  end
+
+  def call(conn, _opts), do: conn
+
+  defp record_and_prepare(conn, ua, browser, os) do
+    visitor_hash = Salt.hash_visitor(conn.remote_ip, ua)
+    {referrer, referrer_source} = extract_referrer(conn)
+    utm = extract_utms(conn)
+    country_code = get_session(conn, "country_code")
+    screen_size = get_session(conn, "analytics_screen_size")
+
+    Analytics.track_pageview(%{
+      pathname: conn.request_path,
+      visitor_hash: visitor_hash,
+      referrer: referrer,
+      referrer_source: referrer_source,
+      utm_source: utm.source,
+      utm_medium: utm.medium,
+      utm_campaign: utm.campaign,
+      country_code: country_code,
+      screen_size: screen_size,
+      browser: browser,
+      os: os
+    })
+
+    conn
+    |> put_session("analytics_visitor_hash", visitor_hash)
+    |> put_session("analytics_browser", browser)
+    |> put_session("analytics_os", os)
+    |> put_session("analytics_referrer", referrer)
+    |> put_session("analytics_referrer_source", referrer_source)
+    |> put_session("analytics_utm_source", utm.source)
+    |> put_session("analytics_utm_medium", utm.medium)
+    |> put_session("analytics_utm_campaign", utm.campaign)
+  end
+
+  # Store session data for the hook even when we skip recording for admins
+  defp prepare_session(conn, ua, browser, os) do
+    visitor_hash = Salt.hash_visitor(conn.remote_ip, ua)
+
+    conn
+    |> put_session("analytics_visitor_hash", visitor_hash)
+    |> put_session("analytics_browser", browser)
+    |> put_session("analytics_os", os)
+  end
+
+  defp admin?(conn) do
+    case conn.assigns[:current_scope] do
+      %{user: %{}} -> true
+      _ -> false
+    end
+  end
+
+  defp extract_referrer(conn) do
+    referrer_url = get_req_header(conn, "referer") |> List.first()
+    {referrer, source} = Referrer.parse(referrer_url)
+
+    # Don't count self-referrals
+    host = conn.host
+
+    if referrer && referrer == host do
+      {nil, nil}
+    else
+      {referrer, source}
+    end
+  end
+
+  defp extract_utms(conn) do
+    params = conn.query_params || %{}
+
+    %{
+      source: Map.get(params, "utm_source"),
+      medium: Map.get(params, "utm_medium"),
+      campaign: Map.get(params, "utm_campaign")
+    }
+  end
+end