Make Quantity name unique among siblings

Remove Quantity domain - will be replaced by configurable per-domain
root Quantity, limiting selection to descendants only

db/migrate/20250104194343_create_quantities.rb

@@ -2,12 +2,12 @@ class CreateQuantities < ActiveRecord::Migration[7.2]
   def change
     create_table :quantities do |t|
       t.references :user, foreign_key: true
-      t.integer :domain
       t.string :name, null: false, limit: 31
       t.text :description
       t.references :parent, foreign_key: {to_table: :quantities}
 
       t.timestamps null: false
     end
+    add_index :quantities, [:user_id, :parent_id, :name], unique: true
   end
 end