Hardening follow-up: N+1 elimination, type validation, diversify fix

Additional fixes from parallel hardening streams:

- exercise/serializers: remove unused WorkoutEquipment import, add prefetch docs
- generator/serializers: N+1 fix in GeneratedWorkoutDetailSerializer (inline workout dict, prefetch-aware supersets)
- generator/services/plan_builder: eliminate redundant .save() after .create() via single create_kwargs dict
- generator/services/workout_generator: proper type-match validation for HIIT/cardio/core/flexibility; fix diversify type count to account for removed entry
- generator/views: request-level caching for get_registered_user helper; prefetch chain for accept_workout
- superset/serializers: guard against dangling FK in SupersetExerciseSerializer
- workout/helpers: use prefetched data instead of re-querying per superset

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

generator/views.py

@@ -275,7 +275,9 @@ def accept_workout(request, workout_id):
     """
     registered_user = get_registered_user(request)
     generated_workout = get_object_or_404(
-        GeneratedWorkout.objects.select_related('workout', 'workout_type'),
+        GeneratedWorkout.objects.select_related('workout', 'workout_type').prefetch_related(
+            'workout__superset_workout__superset_exercises__exercise',
+        ),
         pk=workout_id,
         plan__registered_user=registered_user,
     )