Implemented JPA entities for Wells Fargo task

[AyushiJain18270]

Summary

Implemented the required JPA entities for the Wells Fargo investment management system.

Changes Made

• Added Client entity with a many-to-one relationship with Advisor.
• Added Portfolio entity with a one-to-one relationship with Client.
• Added Security entity with a many-to-one relationship with Portfolio.
• Added appropriate JPA annotations such as @entity, @id, @GeneratedValue, @column, and relationship mappings.
• Included constructors, getters, and setters for all entities.

Testing

• Successfully built and ran the Spring Boot application.
• Verified that the application starts without any errors.