fix: add migration for completed tasks

priv/repo/migrations/20250405121515_add_completed_tasks.exs

@@ -0,0 +1,13 @@
+defmodule Putzplan.Repo.Migrations.AddCompletedTasks do
+  use Ecto.Migration
+
+  def change do
+    create table(:completed_tasks, primary_key: false) do
+      add :id, :uuid, primary_key: true, null: false
+      add :completion, :date, null: false
+
+      add :user_id, references(:users), null: false
+      add :task_id, references(:tasks), null: false
+    end
+  end
+end