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>

exercise/models.py

@@ -1,7 +1,6 @@
 from django.db import models
 from django.conf import settings
 from django.core.validators import MinValueValidator, MaxValueValidator
-from random import randrange
 
 
 DIFFICULTY_CHOICES = [
@@ -49,7 +48,7 @@ class Exercise(models.Model):
     equipment_required = models.CharField(null=True, blank=True, max_length=255)
     muscle_groups = models.CharField(null=True, blank=True, max_length=255)
     synonyms = models.CharField(null=True, blank=True, max_length=255)
-    estimated_rep_duration = models.FloatField(null=True, blank=True, max_length=255)
+    estimated_rep_duration = models.FloatField(null=True, blank=True)
     video_override = models.CharField(null=True, blank=True, max_length=255)
 
     # New fields for workout generation quality