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>

superset/helpers.py

@@ -12,15 +12,26 @@ def get_first_up_superset(workout):
         return first_up_superset
 
 def get_first_up_superset_exercise(superset):
+    try:
+        exercise = Exercise.objects.get(name="First Up")
+    except Exercise.DoesNotExist:
+        exercise = None
+
+    if exercise is None:
+        return None
+
     first_up_superset_exercise = SupersetExercise()
-    first_up_superset_exercise.exercise = Exercise.objects.get(name="First Up")
+    first_up_superset_exercise.exercise = exercise
     first_up_superset_exercise.duration = 15
     first_up_superset_exercise.order = 1
-    first_up_superset_exercise.exercise.side = "Chill"
-    first_up_superset_exercise.exercise.joints_used = ""
-    first_up_superset_exercise.exercise.movement_patterns = ""
-    first_up_superset_exercise.exercise.equipment_required = ""
-    first_up_superset_exercise.exercise.muscle_groups = ""
+    # Build overrides as a dict instead of mutating the ORM object in memory
+    first_up_superset_exercise._display_overrides = {
+        'side': 'Chill',
+        'joints_used': '',
+        'movement_patterns': '',
+        'equipment_required': '',
+        'muscle_groups': '',
+    }
     if superset is not None:
         first_up_superset_exercise.superset = superset
-    return first_up_superset_exercise
+    return first_up_superset_exercise