feat(admin): live redis kill-switch on tunnel actions, sortable columns + CSV export + bulk actions, Node 24 LTS

WS1: pin all Docker stages to node:24.16.0-alpine; add engines node>=20.
WS2: lib/redis.ts gains TTL-backed redisSet, redisDel, setTunnelActive (writes tunnel:active:<sub>=1/0 EX 30, TUNNEL_ACTIVE_TTL override, no-op without REDIS_URL); wired into tunnel active/delete/reassign routes.
WS3: sortable columns, CSV export routes (token excluded), and bulk actions (self-account guard) across users/tunnels/audit admin tables.

package.json

@@ -2,6 +2,9 @@
   "name": "linumiq-web",
   "version": "1.0.0",
   "private": true,
+  "engines": {
+    "node": ">=20"
+  },
   "scripts": {
     "dev": "next dev",
     "build": "next build",