add period comparison deltas to analytics stat cards

Each stat card now shows the percentage change vs the equivalent
previous period (e.g. 30d compares last 30 days vs 30 days before).
Handles zero-baseline with "new" label and caps extreme deltas at
>999%. Seed data extended to 2 years for meaningful 12m comparisons.

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

test/berrypod_web/live/admin/analytics_test.exs

@@ -62,7 +62,7 @@ defmodule BerrypodWeb.Admin.AnalyticsTest do
 
       {:ok, view, _html} = live(conn, ~p"/admin/analytics")
 
-      assert has_element?(view, "rect")
+      assert has_element?(view, "[data-bars]")
     end
 
     test "changes period", %{conn: conn} do