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.

.env.example

@@ -31,3 +31,8 @@ SMTP_ENABLE_STARTTLS=false
 
 # Application variables
 STRIPE_API_KEY=1337
+
+# OpenAI login
+OPENAI_API_KEY=f66dbb5f-9770-4f81-b2ea-eb7370bc9aa5
+OPENAI_BASE_URL=https://api.scaleway.ai/v1
+OPENAI_MODEL=devstral-small-2505