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/deploy.html

@@ -270,7 +270,22 @@
                     {{if .LockedAfterDeploy}}<span class="locked-hint">telepítés után nem módosítható</span>{{end}}
                 </label>
 
-                {{if eq .Type "select"}}
+                {{if eq .Type "subdomain"}}
+                <div class="subdomain-input-group">
+                    <input type="text" id="field-{{.EnvVar}}" name="{{.EnvVar}}"
+                           class="form-control subdomain-input"
+                           value="{{if and $.AlreadyDeployed $.DeployedFieldValues}}{{index $.DeployedFieldValues .EnvVar}}{{else}}{{.Default}}{{end}}"
+                           placeholder="aldomain"
+                           pattern="[a-z0-9]([a-z0-9-]*[a-z0-9])?"
+                           required
+                           {{if $.AlreadyDeployed}}disabled{{end}}
+                           oninput="this.value=this.value.toLowerCase().replace(/[^a-z0-9-]/g,'')">
+                    <span class="subdomain-suffix">.{{$.Domain}}</span>
+                </div>
+                {{if not $.AlreadyDeployed}}
+                <span class="form-hint">Az aldomain telepítés után nem módosítható. Megváltoztatáshoz az alkalmazás eltávolítása és újratelepítése szükséges (minden adat törlődik).</span>
+                {{end}}
+                {{else if eq .Type "select"}}
                 <select id="field-{{.EnvVar}}" name="{{.EnvVar}}" class="form-control"
                         {{if $.AlreadyDeployed}}disabled{{end}}>
                     {{range .Options}}
@@ -510,6 +525,17 @@ document.getElementById('deploy-form').addEventListener('submit', async function
         }
     }
 
+    // Client-side validation: check subdomain format
+    const subdomainField = e.target.querySelector('.subdomain-input');
+    if (subdomainField && !subdomainField.disabled) {
+        const sd = subdomainField.value.trim().toLowerCase();
+        if (!sd || !/^[a-z0-9]([a-z0-9-]*[a-z0-9])?$/.test(sd)) {
+            alert('Az aldomain csak kisbetűket, számokat és kötőjelet tartalmazhat, és nem kezdődhet/végződhet kötőjellel.');
+            subdomainField.focus();
+            return;
+        }
+    }
+
     // Client-side validation: check all required fields
     const requiredFields = e.target.querySelectorAll('input[required], select[required]');
     for (const rf of requiredFields) {