perf: add index on pskr_spots_hourly.last_spot_at for /pskreporter ORDER BY

priv/repo/migrations/20260716121941_add_last_spot_at_index.exs

@@ -0,0 +1,7 @@
+defmodule Microwaveprop.Repo.Migrations.AddLastSpotAtIndex do
+  use Ecto.Migration
+
+  def change do
+    create index(:pskr_spots_hourly, ["last_spot_at DESC NULLS LAST"])
+  end
+end