feat(chat): add username to users

And use it instead of hacky email transformation.

priv/repo/migrations/20250205000835_alter_users_add_username.exs

@@ -0,0 +1,25 @@
+defmodule Slax.Repo.Migrations.AlterUsersAddUsername do
+  use Ecto.Migration
+
+  def change do
+    alter table(:users) do
+      add :username, :citext
+    end
+
+    query = """
+    update
+      users
+    set
+      username = initcap(substring(email from '^[^@]+'))
+    ;
+    """
+
+    execute query, ""
+
+    alter table(:users) do
+      modify :username, :citext, null: false, from: {:citext, null: true}
+    end
+
+    create unique_index(:users, :username)
+  end
+end