v0.27.0 — user-configurable app subdomains

Users can now customize the subdomain for each app during deployment
instead of using a fixed value. The deploy page shows an editable text
input with the default pre-filled and the base domain as a suffix.

New "subdomain" deploy field type with DNS-safe format validation,
reserved name blocklist, and uniqueness check across deployed stacks.
Locked after deploy — changing requires Remove + Redeploy.

Backward compatible: InjectMissingFields() auto-fills SUBDOMAIN from
.felhom.yml defaults for existing deployed apps on next sync/restart.

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

controller/internal/web/templates/stacks.html

@@ -24,9 +24,10 @@
                      alt="" onerror="this.onerror=function(){this.style.display='none'};this.src='{{logoPNGURL .Meta.Slug}}'">
                 <div>
                     <h3>{{.Meta.DisplayName}}</h3>
-                    {{if .Meta.Subdomain}}
-                    <a class="subdomain-link" href="https://{{.Meta.Subdomain}}.{{$.Domain}}" target="_blank">
-                        {{.Meta.Subdomain}}.{{$.Domain}} ↗
+                    {{$subdomain := index $.Subdomains .Name}}
+                    {{if $subdomain}}
+                    <a class="subdomain-link" href="https://{{$subdomain}}.{{$.Domain}}" target="_blank">
+                        {{$subdomain}}.{{$.Domain}} ↗
                     </a>
                     {{end}}
                 </div>