harden security and add public TLS scaffold

- require ADMIN_PASSWORD (no default), remove CORS
- close public DB port, move DB credentials to .env (DB_PASSWORD)
- fix HTML escaping, add helmet + sec headers (no CSP due to inline scripts)
- rate limit public routes by IP (express-rate-limit)
- validate restore data and confine file unlinking to uploads/
- block dangerous upload extensions, 30MB per-entry limit, SVG not served inline
- return 400 on unknown group_id in POST /api/entries
- add commented Caddy/Let's Encrypt reverse-proxy scaffold + Caddyfile.example
- update README

.gitignore

@@ -22,3 +22,6 @@ node_modules/
 tmp/
 .DS_Store
 Thumbs.db
+
+# --- Internal audit (not for commit) ---
+SECURITY_AUDIT.md