fix: Add slug column to parties table and update seed data

- Added slug column to parties table via migration
- Updated seed data to properly assign slug values to Party records
- Fixed NoMethodError when assigning slug attribute in seed file

This resolves the ActiveModel::UnknownAttributeError that occurred
when trying to set the slug attribute on Party model instances.

db/migrate/20250823145902_create_parties.rb

@@ -2,6 +2,7 @@ class CreateParties < ActiveRecord::Migration[8.0]
   def change
     create_table :parties do |t|
       t.string :name, null: false
+      t.string :slug, null: false
       t.string :image, null: true
       t.text :description, null: false
       t.integer :state, default: 0, null: false