valia-sites: take over stem95su DNS (manage_dns=true) — cutover half 2

Creates the public proxied CNAME stem95su -> stem95su.pages.dev and
adds the internal split-horizon entry via the valia-sites-dns
ConfigMap (the sync's update pass repoints the existing internal
record). Completes the ADR-0018 cutover; the old in-cluster serving
stack is retired in a follow-up.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

stacks/valia-sites/main.tf

@@ -31,7 +31,7 @@ locals {
       folder_id  = "1cmOI2jRyBJdnrVPgbr4kx2cx_4DY6pm_" # "claude" — 95. СУ STEM board
       src_path   = "stem claude/files"
       entry_file = "stem_board.html"
-      manage_dns = false # flipped true in the cutover commit (record still owned by stacks/stem95su ingress_factory)
+      manage_dns = true
     }
   }