feat(chat): add schema to handle room membership

priv/repo/migrations/20241117132804_create_room_memberships.exs

@@ -0,0 +1,16 @@
+defmodule Slax.Repo.Migrations.CreateRoomMemberships do
+  use Ecto.Migration
+
+  def change do
+    create table(:room_memberships) do
+      add :user_id, references(:users, on_delete: :delete_all), null: false
+      add :room_id, references(:rooms, on_delete: :delete_all), null: false
+
+      timestamps(type: :utc_datetime)
+    end
+
+    create index(:room_memberships, [:user_id])
+    create index(:room_memberships, [:room_id])
+    create unique_index(:room_memberships, [:user_id, :room_id])
+  end
+end