replace analytics double-count prevention with buffer supersede

The Plug records a pageview with a known ID (plug_ref) into the ETS
buffer. When JS connects, the LiveView hook supersedes that event by
ID and records its own with full data (screen_size from connect params).
If JS never connects, the Plug's event flushes normally after 10s.

Also fixes: admin browsing no longer leaks product_view events — the
Plug now sets no analytics session data for admins, so all downstream
visitor_hash guards naturally filter them out.

Replaces the previous time-based skip logic which was brittle and
race-prone. The supersede approach is deterministic and handles both
the ETS buffer and already-flushed DB cases.

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

test/berrypod_web/plugs/analytics_test.exs

@@ -42,8 +42,6 @@ defmodule BerrypodWeb.Plugs.AnalyticsTest do
       :timer.sleep(50)
 
       count_after = Repo.aggregate(Event, :count)
-      # POST to /checkout shouldn't create a pageview event via the plug
-      # (it may fail with a redirect, but the plug should have skipped)
       assert count_after == count_before
     end
 
@@ -71,6 +69,17 @@ defmodule BerrypodWeb.Plugs.AnalyticsTest do
       assert get_session(conn, "analytics_browser") == "Firefox"
     end
 
+    test "stores plug_ref in session for buffer supersede", %{conn: conn} do
+      conn =
+        conn
+        |> put_req_header("user-agent", "Mozilla/5.0 Chrome/120.0")
+        |> get(~p"/")
+
+      plug_ref = get_session(conn, "analytics_plug_ref")
+      assert is_binary(plug_ref)
+      assert String.length(plug_ref) == 36
+    end
+
     test "extracts referrer", %{conn: conn} do
       conn
       |> put_req_header("user-agent", "Mozilla/5.0 Chrome/120.0")