basic admin endgame view and add (#3)

Reviewed-on: #3

priv/repo/migrations/20251127164545_create_endgames.exs

@@ -0,0 +1,23 @@
+defmodule Chesstrainer.Repo.Migrations.CreateEndgames do
+  use Ecto.Migration
+
+  def change do
+    create table(:endgames, primary_key: false) do
+      add :id, :binary_id, primary_key: true
+      add :fen, :string
+      add :color, :string
+      add :key, :string
+      add :message, :text
+      add :notes, :text
+      add :result, :string
+      add :rating, :integer, default: 1500
+      add :rating_deviation, :integer, default: 350
+      add :times_attempted, :integer, default: 0
+      add :times_solved, :integer, default: 0
+
+      timestamps(type: :utc_datetime)
+    end
+
+    create unique_index(:endgames, [:fen])
+  end
+end