fix: address code review — narrow exception types, computed dkimLiveText getter

Agent-Logs-Url: https://github.com/christianlouis/dmarq/sessions/19d17518-732d-4644-889b-cc63256e19b1

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

backend/app/templates/domain_details.html

@@ -160,7 +160,7 @@
                             <span x-show="!dns.dkim" class="inline-flex h-2 w-2 rounded-full bg-red-500"></span>
                         </h3>
                         <div class="bg-muted p-2 rounded text-sm overflow-x-auto font-mono"
-                             x-text="dns.dkim ? (dns.dkimSelectors ? 'selector: ' + dns.dkimSelectors : 'Verified') : 'No DKIM record found for configured selectors'">-</div>
+                             x-text="dkimLiveText">-</div>
                     </div>
                     <!-- DKIM Selector Management -->
                     <div class="border rounded-lg p-4">
@@ -421,6 +421,12 @@ function domainDetailsApp(domainId) {
             });
         },
 
+        get dkimLiveText() {
+            if (!this.dns.dkim) return 'No DKIM record found for configured selectors';
+            if (this.dns.dkimSelectors) return 'selector: ' + this.dns.dkimSelectors;
+            return 'Verified';
+        },
+
         async fetchDomainStats() {
             try {
                 const response = await fetch(`/api/v1/domains/${this.domainId}/stats`);