fix: scope subscription slug uniqueness per subscriber

[mckenziearts]

Previously, subscription slugs were unique globally across all subscribers, causing slugs to increment unnecessarily (e.g., "enterprise", "enterprise-1", "enterprise-2").

Now, slug uniqueness is scoped to the subscriber (subscriber_type + subscriber_id), allowing multiple subscribers to have subscriptions with the same slug.

Problem

Subscription slugs were unique globally across all subscribers. When multiple subscribers subscribed to the same plan, slugs would increment unnecessarily:

subscriber_id	slug
1	enterprise
2	enterprise-1
3	enterprise-2

Solution

Scope the slug uniqueness check to the subscriber using Spatie Sluggable's extraScope() method. Now each subscriber can have a subscription with the same slug:

subscriber_id	slug
1	enterprise
2	enterprise
3	enterprise

Changes

• Updated getSlugOptions() in Subscription model to scope uniqueness by subscriber_type and subscriber_id