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/services/plan_builder.py

@@ -96,26 +96,27 @@ class PlanBuilder:
 
                     order = ex_spec.get('order', 1)
 
-                    superset_exercise = SupersetExercise.objects.create(
-                        superset=superset,
-                        exercise=exercise_obj,
-                        order=order,
-                    )
+                    # Build kwargs for create, including optional fields,
+                    # so we don't need a separate .save() after .create().
+                    create_kwargs = {
+                        'superset': superset,
+                        'exercise': exercise_obj,
+                        'order': order,
+                    }
 
-                    # Assign optional fields exactly like add_workout does
                     if ex_spec.get('weight') is not None:
-                        superset_exercise.weight = ex_spec['weight']
+                        create_kwargs['weight'] = ex_spec['weight']
 
                     if ex_spec.get('reps') is not None:
-                        superset_exercise.reps = ex_spec['reps']
+                        create_kwargs['reps'] = ex_spec['reps']
                         rep_duration = exercise_obj.estimated_rep_duration or 3.0
                         superset_total_time += ex_spec['reps'] * rep_duration
 
                     if ex_spec.get('duration') is not None:
-                        superset_exercise.duration = ex_spec['duration']
+                        create_kwargs['duration'] = ex_spec['duration']
                         superset_total_time += ex_spec['duration']
 
-                    superset_exercise.save()
+                    SupersetExercise.objects.create(**create_kwargs)
 
                 # ---- 4. Update superset estimated_time ----
                 # Store total time including all rounds and rest between rounds