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/section_builders.py

@@ -0,0 +1,68 @@
+import random
+from typing import Iterable, Optional
+
+
+def section_exercise_count(section: str, fitness_level: int, rng=random) -> int:
+    """Return section exercise count range by fitness level."""
+    level = fitness_level or 2
+    if section == 'warmup':
+        if level <= 1:
+            return rng.randint(5, 7)
+        if level >= 3:
+            return rng.randint(3, 5)
+        return rng.randint(4, 6)
+    if section == 'cooldown':
+        if level <= 1:
+            return rng.randint(4, 5)
+        if level >= 3:
+            return rng.randint(2, 3)
+        return rng.randint(3, 4)
+    raise ValueError(f'Unknown section: {section}')
+
+
+def rounded_duration(
+    raw_duration: int,
+    *,
+    min_duration: int,
+    duration_multiple: int,
+) -> int:
+    """Round duration to configured multiple and clamp to minimum."""
+    return max(min_duration, round(raw_duration / duration_multiple) * duration_multiple)
+
+
+def build_duration_entries(
+    exercises: Iterable,
+    *,
+    duration_min: int,
+    duration_max: int,
+    min_duration: int,
+    duration_multiple: int,
+    rng=random,
+) -> list[dict]:
+    """Build ordered duration entries from exercises."""
+    entries = []
+    for idx, ex in enumerate(exercises, start=1):
+        duration = rng.randint(duration_min, duration_max)
+        entries.append({
+            'exercise': ex,
+            'duration': rounded_duration(
+                duration,
+                min_duration=min_duration,
+                duration_multiple=duration_multiple,
+            ),
+            'order': idx,
+        })
+    return entries
+
+
+def build_section_superset(name: str, entries: list[dict]) -> Optional[dict]:
+    """Build a single-round warmup/cooldown superset payload."""
+    if not entries:
+        return None
+    return {
+        'name': name,
+        'rounds': 1,
+        'rest_between_rounds': 0,
+        'exercises': entries,
+    }
+