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>

.env.example

@@ -7,6 +7,9 @@ VERSION=latest
 # Public access
 PUBLIC_HOST=localhost
 PUBLIC_PROTOCOL=https
+# Auth domain (Logto). Defaults to PUBLIC_HOST for single-domain setups.
+# Set to a separate subdomain (e.g. auth.cameleer.io) to split auth from the app.
+# AUTH_HOST=localhost
 
 # Ports
 HTTP_PORT=80