security: QA Council audit fixes

- Remove JWT secret fallbacks (fail on startup if missing)
- Hash refresh tokens with SHA-256 before DB storage
- Add Zod validation to bulk-approve, bulk-reject, update-user, copy-week
- Add global API rate limiting (100 req/15min per IP)
- Fix nginx X-XSS-Protection header (align with Helmet)
- Remove --accept-data-loss from Dockerfile CMD
- Create .gitignore to protect secrets from commits
- Secure .env file permissions (chmod 600)

docker/nginx/default.conf

@@ -15,7 +15,7 @@ server {
     # Security headers
     add_header X-Frame-Options "SAMEORIGIN" always;
     add_header X-Content-Type-Options "nosniff" always;
-    add_header X-XSS-Protection "1; mode=block" always;
+    add_header X-XSS-Protection "0" always;
     add_header Referrer-Policy "strict-origin-when-cross-origin" always;
 
     # Gzip compression