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>

workout/helpers.py

@@ -30,10 +30,14 @@ def create_all_exercise_list_for_workout(workout):
     data["audio_queues"] = audio_queues
     all_superset_exercise.append(data)
 
-    supersets = Superset.objects.filter(workout=workout).order_by('order')
+    # Fix #17: N+1 - add prefetch_related to avoid per-superset queries
+    supersets = Superset.objects.filter(workout=workout).order_by('order').prefetch_related(
+        'supersetexercise_set__exercise'
+    )
     order = 2
     for superset_count, superset in enumerate(supersets):
-        supersetExercises = SupersetExercise.objects.filter(superset=superset).order_by('order')
+        # Use prefetched data instead of re-querying (N+1 fix)
+        supersetExercises = sorted(superset.supersetexercise_set.all(), key=lambda se: se.order)
         for x in range(superset.rounds):
             for exercise_idx, exercise in enumerate(supersetExercises):
                 exercise.order = order
@@ -68,7 +72,9 @@ def create_all_exercise_list_for_workout(workout):
 
                     elif len(supersets) > superset_count+1:
                         next_superset = supersets[superset_count+1]
-                        next_supersetExercises = SupersetExercise.objects.filter(superset=next_superset).order_by('order').first()
+                        # Use prefetched data instead of re-querying
+                        next_superset_exercises = sorted(next_superset.supersetexercise_set.all(), key=lambda se: se.order)
+                        next_supersetExercises = next_superset_exercises[0] if next_superset_exercises else None
                         
                         next_up_data = {
                             "audio_url": next_supersetExercises.exercise.audio_url().lower(),