Resolve N+1 Queries & Eager Fetching

[viceda-s]

Problem: WorkoutService suffers from massive N+1 queries during writes (looping .findById), and implicit Eager fetching causes N+1 on reads.

Acceptance Criteria:

• Refactor WorkoutService.buildWorkoutExercises to fetch all exercises in a single query using exerciseRepository.findAllById(ids).
• Change all @ManyToOne relationships (WorkoutPlan.owner, WorkoutExercise.exercise) to use fetch = FetchType.LAZY.
• Update the WorkoutPlanRepository to use JOIN FETCH queries when loading plans to prevent lazy-loading N+1s during API serialization.

[viceda-s]

Resolved N+1 queries by implementing JOIN FETCH via @EntityGraph in repositories, changing default eager fetching to LAZY in domain entities, and batch-fetching exercises in WorkoutService. Unit tests updated. Tests passed.