feat(server): session-based auth plug + login controller/template

server/lib/server_web/plugs/require_auth.ex

@@ -0,0 +1,19 @@
+defmodule ServerWeb.Plugs.RequireAuth do
+  @moduledoc "Redirects to /login unless the session is authenticated."
+
+  import Plug.Conn
+  import Phoenix.Controller
+
+  def init(opts), do: opts
+
+  def call(conn, _opts) do
+    if get_session(conn, :authenticated) do
+      conn
+    else
+      conn
+      |> put_flash(:error, "Please sign in.")
+      |> redirect(to: "/login")
+      |> halt()
+    end
+  end
+end