Add backend for group invitations

For #887

priv/repo/migrations/20251027133219_create_invitations.exs

@@ -0,0 +1,15 @@
+defmodule Mobilizon.Storage.Repo.Migrations.CreateInvitations do
+  use Ecto.Migration
+
+  def change do
+    create table(:invitations) do
+      add(:label, :string, default: "", null: false)
+      add(:token, :string, default: fragment("gen_random_uuid()"), null: false)
+      add(:group_id, references(:actors, on_delete: :delete_all), null: false)
+
+      timestamps()
+    end
+
+    create(unique_index(:invitations, [:token]))
+  end
+end