fix(status): read the public Kuma page, with notices scoped by a prefix

The banner read Kuma's "matrix" status page, which was merged into "public"
on 2026-10-11, so it has returned 404 since and the banner stayed silent
(by design it fails quiet). Point it at "public", whose Matrix group is now
split into Homeserver / Voice calls / Discord.

That page is shared with other services, so add an optional noticePrefix:
only incidents and maintenance titled "Matrix: ..." reach Lotus users, shown
without the prefix. A media server's maintenance can't say messages may be
interrupted, and the Discord group can't raise a banner at all.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

config.json

@@ -16,7 +16,8 @@
   "statusPages": {
     "matrix.lotusguild.org": {
       "url": "https://isitup.lotusguild.org",
-      "slug": "matrix",
+      "slug": "public",
+      "noticePrefix": "Matrix:",
       "groups": {
         "homeserver": "Homeserver",
         "calls": "Voice calls",