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/app_info.html

@@ -10,7 +10,7 @@
         {{if .Stack.Deployed}}
             <span class="stack-state-badge state-{{stateColor .Stack.State}}">{{stateLabel .Stack.State}}</span>
             {{if .Stack.Orphaned}}<span class="badge badge-orphaned">Elavult</span>{{end}}
-            <a href="https://{{.Meta.Subdomain}}.{{.Domain}}" target="_blank" class="btn btn-sm btn-outline">Megnyitás ↗</a>
+            {{if .EffectiveSubdomain}}<a href="https://{{.EffectiveSubdomain}}.{{.Domain}}" target="_blank" class="btn btn-sm btn-outline">Megnyitás ↗</a>{{end}}
             <a href="/stacks/{{.Stack.Name}}/logs" class="btn btn-sm btn-outline">Napló</a>
             {{if .Stack.Orphaned}}
                 <button class="btn btn-sm btn-danger" onclick="deleteOrphanStack('{{.Stack.Name}}')">Törlés</button>