Add health controller

Fly.io uses '/health' to determine the application health status.

lib/music_library_web/controllers/health_controller.ex

@@ -0,0 +1,19 @@
+defmodule MusicLibraryWeb.HealthController do
+  use MusicLibraryWeb, :controller
+
+  alias MusicLibrary.Repo
+
+  def index(conn, _params) do
+    case Repo.query("SELECT 1") do
+      {:ok, _} ->
+        conn
+        |> put_resp_content_type("text/html")
+        |> send_resp(200, "App is running")
+
+      {:error, _} ->
+        conn
+        |> put_resp_content_type("text/html")
+        |> send_resp(500, "App is not running")
+    end
+  end
+end