Persist Quantity :depth instead of computing it on the fly

db/migrate/20250104194343_create_quantities.rb

@@ -5,6 +5,7 @@ class CreateQuantities < ActiveRecord::Migration[7.2]
       t.string :name, null: false, limit: 31
       t.text :description
       t.references :parent, foreign_key: {to_table: :quantities}
+      t.integer :depth, null: false, default: 0
 
       t.timestamps null: false
     end

db/schema.rb

@@ -16,6 +16,7 @@ ActiveRecord::Schema[7.2].define(version: 2025_01_04_194343) do
     t.string "name", limit: 31, null: false
     t.text "description"
     t.bigint "parent_id"
+    t.integer "depth", default: 0, null: false
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
     t.index ["parent_id"], name: "index_quantities_on_parent_id"