fix: use configured domain in CoreDNS Corefile generation

Two bugs caused DNS to fail when the domain name changes:
1. generate_corefile() hardcoded 'cell' as the zone name instead of
   using the configured domain — on startup it would silently reset any
   domain change back to 'cell'
2. apply_domain() regex replaced ALL non-dot zones (including local.cell)
   with the new domain → duplicate zone blocks → CoreDNS crash

Fix: add a domain parameter to generate_corefile/apply_all_dns_rules,
add _configured_domain() helper in app.py, and delegate Corefile updates
in apply_domain() to generate_corefile() so the logic is in one place.
Also parameterise SERVICE_HOSTS ACL entries via the domain argument.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

config/dns/Corefile

@@ -5,12 +5,12 @@
     health
 }
 
-cell {
-    file /data/cell.zone
+dev {
+    file /data/dev.zone
     log
 }
 
-local.cell {
+local.dev {
     file /data/local.zone
     log
 }