Fix N+1 Query Performance Issue in Vets Endpoint-created-by-agentic

[shaykeren]

The Vets endpoint (/vets.html) is experiencing an N+1 query performance issue due to eager loading of specialties for each vet.

Problem:

• The Vet entity uses EAGER fetching for the specialties relationship
• This causes a separate query for each vet's specialties
• No database indexes on the join table columns

Solution:

1. Changed fetch type to LAZY with @batchsize optimization
2. Added @EntityGraph for efficient loading
3. Added database indexes on:
   • vet_specialties.vet_id
   • vet_specialties.specialty_id
   • specialties.id

Related PR: #85