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>

lib/berrypod/analytics/buffer.ex

@@ -55,6 +55,20 @@ defmodule Berrypod.Analytics.Buffer do
     GenServer.cast(__MODULE__, {:record, attrs})
   end
 
+  @doc """
+  Removes the Plug's initial pageview so the hook can replace it.
+
+  Looks in the ETS buffer first. If the event was already flushed to the DB
+  (the 10s timer can fire between Plug record and hook connect), deletes it
+  from there instead. Either way, the hook then records its own event with
+  full data (screen_size etc.).
+
+  If JS never connects, the Plug's event stays and flushes normally.
+  """
+  def supersede(ref) when is_binary(ref) do
+    GenServer.cast(__MODULE__, {:supersede, ref})
+  end
+
   # ── GenServer callbacks ──
 
   @impl true
@@ -75,7 +89,7 @@ defmodule Berrypod.Analytics.Buffer do
     event =
       attrs
       |> Map.put(:session_hash, session_hash)
-      |> Map.put(:id, Ecto.UUID.generate())
+      |> Map.put_new(:id, Ecto.UUID.generate())
       |> Map.put(:inserted_at, DateTime.truncate(now, :second))
 
     counter = state.counter + 1
@@ -84,6 +98,29 @@ defmodule Berrypod.Analytics.Buffer do
     {:noreply, %{state | counter: counter, sessions: sessions}}
   end
 
+  @impl true
+  def handle_cast({:supersede, ref}, state) do
+    found_in_ets =
+      state.table
+      |> :ets.tab2list()
+      |> Enum.reduce(false, fn {key, event}, found ->
+        if Map.get(event, :id) == ref do
+          :ets.delete(state.table, key)
+          true
+        else
+          found
+        end
+      end)
+
+    # If the 10s flush already moved it to the DB, delete it there
+    unless found_in_ets do
+      import Ecto.Query
+      Repo.delete_all(from(e in Event, where: e.id == ^ref))
+    end
+
+    {:noreply, state}
+  end
+
   @impl true
   def handle_info(:flush, state) do
     state = flush_events(state)