Fix case-insensitive auth and harden for production

- Fix login/register to treat usernames case-insensitively (HeagBoKaT == heagbokat) using LOWER() in SQL with JS .toLowerCase() for Unicode safety
- Add helmet for HTTP security headers and express-rate-limit on auth endpoints (20 req / 15 min) to prevent brute-force
- Lock CORS to ALLOWED_ORIGIN env var; warn if JWT_SECRET is the default dev key
- Serve built client/dist from Express so a single PM2 process handles everything
- Add deploy.sh, README.md with full first-run and auto-restart instructions, and server/.env.example

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

deploy.sh

@@ -0,0 +1,20 @@
+#!/bin/bash
+set -e
+
+echo "==> Pulling latest changes..."
+git pull
+
+echo "==> Installing server dependencies..."
+npm install --prefix server
+
+echo "==> Installing client dependencies..."
+npm install --prefix client
+
+echo "==> Building client..."
+npm run build
+
+echo "==> Restarting PM2 process..."
+sudo pm2 restart cats
+sudo pm2 save
+
+echo "==> Done! Site updated at http://cats.heagbokat-server.ru"