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>

.env.example

@@ -0,0 +1,25 @@
+# Django core
+DEBUG=false
+SECRET_KEY=your-secret-key-here
+
+# Allowed hosts (comma-separated)
+ALLOWED_HOSTS=yourdomain.com
+
+# CORS (comma-separated origins)
+CORS_ALLOWED_ORIGINS=https://yourdomain.com
+
+# Database (used when DATABASE_URL is not set)
+DB_NAME=werkout
+DB_USER=werkout
+DB_PASSWORD=your-db-password
+DB_HOST=db
+DB_PORT=5432
+
+# Database URL (overrides individual DB_* vars when set)
+DATABASE_URL=postgres://user:password@db:5432/werkout
+
+# Redis
+REDIS_URL=redis://redis:6379
+
+# HTTPS redirect (set to false if behind a reverse proxy that handles SSL)
+SECURE_SSL_REDIRECT=true