fix(demo): lock removing a Steam link on the demo (DM27)

The step-5 walk found demo-player could remove its stand-in Steam link from
its Rust page, and staff could remove it from Admin -> Users. That link is
what the player tour's Rust pages, rewards and grants hang on.

- module.json demoLocks gains DELETE /player/rust/links/:steamId.
- The staff route, DELETE /admin/users/:id/rust/links/:steamId, serves under
  core's users mount, which a module's demoLocks may not name, so its handler
  refuses with DEMO_LOCKED behind ctx.demoMode before the link is touched.

Off the demo both behave as before. Checked on a real core with the demo
seed: both answer 403 DEMO_LOCKED and the link stays.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E14m6SuuY6i1vASFeGDBeY

server/router/admin/usersRust.controller.js

@@ -8,6 +8,7 @@
 const crypto = require('node:crypto')
 
 const core = require('../../core')
+const demo = require('../../demo')
 
 const backpacks = require('../../model/backpacks/backpacks.model')
 const economics = require('../../model/economics/economics.model')
@@ -54,6 +55,10 @@ async function removeLink(req, res) {
   const { steamId } = req.params
   const userId = req.params.id
 
+  // On the demo the player's own unlink is locked by demoLocks; this route serves
+  // under core's users mount, which a module may not lock by route, so it asks here.
+  if (demo.isDemo()) return demo.refuse(res)
+
   try {
     const removed = await links.unlinkOwned(steamId, userId)