Batch add and update group specialties

Adds new specialty associations and/or updates existing specialty associations for a group in a single request. New entries create group–specialty records; update entries modify existing associations (effective/termination dates, specialty linkage). Use this endpoint for bulk specialty management — for single-specialty additions use POST /groups/{id}/specialties, and for date-only updates to a single specialty use PUT /groups/{id}/specialties. Permission note: if any update entry includes a non-blank newTenantSpecialtyId (reassigning the specialty), the caller must also hold the group:correct:specialty permission — such a request without that permission returns 403. Pure additions or date/isPrimary-only updates only require the standard group:update permission.