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/services/workout_generation/recovery.py

@@ -0,0 +1,14 @@
+def is_recovery_exercise(ex) -> bool:
+    """True for warmup/cooldown-style recovery/stretch exercises."""
+    if ex is None:
+        return False
+    name = (getattr(ex, 'name', '') or '').lower()
+    patterns = (getattr(ex, 'movement_patterns', '') or '').lower()
+    if 'stretch' in name:
+        return True
+    blocked = (
+        'mobility - static', 'static stretch', 'yoga',
+        'cool down', 'cooldown', 'breathing', 'massage',
+    )
+    return any(token in patterns for token in blocked)
+