Add index on competitions end_date

We filter old competitions for data removal based on end_date,
so this index is to make that query more efficient.

priv/repo/migrations/20241202060152_add_competitions_end_date_index.exs

@@ -0,0 +1,7 @@
+defmodule WcaLive.Repo.Migrations.AddCompetitionsEndDateIndex do
+  use Ecto.Migration
+
+  def change do
+    create index(:competitions, [:end_date])
+  end
+end