Remove Lombok @Data from Entities

[viceda-s]

Problem: @Data auto-generates hashCode() and toString() that can trigger lazy collections, causing severe performance impacts or StackOverflowError in bidirectional relationships.

Acceptance Criteria:

• Replace @Data with @Getter and @Setter on User, WorkoutPlan, WorkoutExercise, and Exercise.
• Only implement equals() and hashCode() strictly based on the primary key or natural business keys if necessary.

[viceda-s]

Lombok @Data annotations have been successfully replaced with @Getter and @Setter across all JPA domain entities. This resolves the implicit proxy-loading risks caused by generated hashCode(), equals(), and toString() methods, ensuring stable entity state management. Tests passed. Closing issue.