Move migration defaults to ActiveRecord migration strategy

Remove redundant defaults

db/migrate/20250104194343_create_quantities.rb

@@ -1,4 +1,4 @@
-class CreateQuantities < ActiveRecord::Migration[7.2]
+class CreateQuantities < ActiveRecord::Migration[8.1]
   def change
     create_table :quantities do |t|
       t.references :user, foreign_key: {on_delete: :cascade}
@@ -6,12 +6,12 @@ class CreateQuantities < ActiveRecord::Migration[7.2]
       t.text :description
       t.references :parent, foreign_key: {to_table: :quantities, on_delete: :cascade}
 
-      t.timestamps null: false
+      t.timestamps
 
       # Caches; can be computed from other attributes
       t.integer :depth, null: false, default: 0
       t.string :pathname, null: false, limit: 511
     end
-    add_index :quantities, [:user_id, :parent_id, :name], unique: true
+    add_index :quantities, [:user_id, :parent_id, :name]
   end
 end