Applications: link to placement provider; better profile fields when the provider is nested

New: placement-provider relationship on applications

Application resources now expose a placement-provider relationship.

• It points to the placement provider tied to the application’s license (same provider you work with on the placement providers endpoints).
• Use GET /api/v1/placement_providers/:id with the relationship’s id when you need the full placement provider payload, related members, licenses, and other include options.

The public schema for applications documents this relationship for validation and reference.