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/management/commands/recalculate_workout_times.py

@@ -46,16 +46,18 @@ class Command(BaseCommand):
         dry_run = options['dry_run']
         rest_between_rounds = options['rest']
 
-        workouts = Workout.objects.all()
+        workouts = Workout.objects.prefetch_related(
+            'superset_workout__superset_exercises__exercise'
+        ).all()
         total = workouts.count()
         updated = 0
 
         for workout in workouts:
-            supersets = Superset.objects.filter(workout=workout).order_by('order')
+            supersets = workout.superset_workout.all().order_by('order')
             workout_total_time = 0
 
             for ss in supersets:
-                exercises = SupersetExercise.objects.filter(superset=ss)
+                exercises = ss.superset_exercises.all()
                 active_time = 0.0
 
                 for se in exercises: