Codebase hardening: 102 fixes across 35+ files

Deep audit identified 106 findings; 102 fixed, 4 deferred. Covers 8 areas:

- Settings & deploy: env-gated DEBUG/SECRET_KEY, HTTPS headers, gunicorn, celery worker
- Auth (registered_user): password write_only, request.data fixes, transaction safety, proper HTTP status codes
- Workout app: IDOR protection, get_object_or_404, prefetch_related N+1 fixes, transaction.atomic
- Video/scripts: path traversal sanitization, HLS trigger guard, auth on cache wipe
- Models (exercise/equipment/muscle/superset): null-safe __str__, stable IDs, prefetch support
- Generator views: helper for registered_user lookup, logger.exception, bulk_update, transaction wrapping
- Generator core (rules/selector/generator): push-pull ratio, type affinity normalization, modality checks, side-pair exact match, word-boundary regex, equipment cache clearing
- Generator services (plan_builder/analyzer/normalizer): transaction.atomic, muscle cache, bulk_update, glutes classification fix

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

generator/serializers.py

@@ -335,7 +335,11 @@ class GeneratedWorkoutDetailSerializer(serializers.ModelSerializer):
 
     def get_supersets(self, obj):
         if obj.workout:
-            superset_qs = Superset.objects.filter(workout=obj.workout).order_by('order')
+            superset_qs = Superset.objects.filter(
+                workout=obj.workout
+            ).prefetch_related(
+                'supersetexercise_set__exercise',
+            ).order_by('order')
             return SupersetSerializer(superset_qs, many=True).data
         return []