feat: split auth domain — Logto gets dedicated AUTH_HOST

Support separate auth domain (e.g. auth.cameleer.io) for Logto while
keeping the SaaS app on PUBLIC_HOST (e.g. app.cameleer.io). AUTH_HOST
defaults to PUBLIC_HOST for backward-compatible single-domain setups.

- Logto routing: Host(AUTH_HOST) replaces PathPrefix('/') catch-all
- Root redirect moved from traefik-dynamic.yml to Docker labels with
  Host(PUBLIC_HOST) scope so it doesn't intercept auth domain
- Self-signed cert generates SANs for both domains
- Bootstrap Host header uses AUTH_HOST for Logto endpoint validation
- Spring issuer-uri and oidcissueruri use new authhost property
- Both installers (sh + ps1) prompt for AUTH_HOST in expert mode

Local dev: AUTH_HOST=auth.localhost (resolves to 127.0.0.1, no hosts file)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/logto-bootstrap.sh

@@ -32,6 +32,7 @@ SAAS_ADMIN_PASS="${SAAS_ADMIN_PASS:-admin}"
 
 # Redirect URIs (derived from PUBLIC_HOST and PUBLIC_PROTOCOL)
 HOST="${PUBLIC_HOST:-localhost}"
+AUTH="${AUTH_HOST:-$HOST}"
 PROTO="${PUBLIC_PROTOCOL:-https}"
 SPA_REDIRECT_URIS="[\"${PROTO}://${HOST}/platform/callback\"]"
 SPA_POST_LOGOUT_URIS="[\"${PROTO}://${HOST}/platform/login\",\"${PROTO}://${HOST}/platform/\"]"
@@ -47,8 +48,9 @@ if [ "$BOOTSTRAP_LOCAL" = "true" ]; then
   HOST_ARGS=""
   ADMIN_HOST_ARGS=""
 else
-  HOST_ARGS="-H Host:${HOST}"
-  ADMIN_HOST_ARGS="-H Host:${HOST}:3002 -H X-Forwarded-Proto:https"
+  # Logto validates Host header against its ENDPOINT, which uses AUTH_HOST
+  HOST_ARGS="-H Host:${AUTH}"
+  ADMIN_HOST_ARGS="-H Host:${AUTH}:3002 -H X-Forwarded-Proto:https"
 fi
 
 # Install jq + curl if not already available (deps are baked into cameleer-logto image)