feat(frontend): replace all name concatenation with displayName

- Add displayName default method to PersonSummaryDTO
- Update native SQL queries to include title, person_type columns
- Add getInitials() utility to personFormat.ts
- Update abbreviateName/abbreviateCompact for nullable firstName
- Replace firstName+lastName concatenation with displayName in all
  person-displaying components and server load files
- Regenerate API types with displayName on Person and PersonSummaryDTO

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

frontend/src/routes/briefwechsel/CorrespondentSuggestionsDropdown.svelte

@@ -4,8 +4,9 @@ import { clickOutside } from '$lib/actions/clickOutside';
 
 interface Correspondent {
 	id: string;
-	firstName: string;
+	firstName?: string | null;
 	lastName: string;
+	displayName: string;
 }
 
 interface Props {
@@ -41,7 +42,9 @@ function handleKeydown(event: KeyboardEvent, container: HTMLElement) {
 }
 
 function getInitials(person: Correspondent): string {
-	return `${person.firstName.charAt(0)}${person.lastName.charAt(0)}`.toUpperCase();
+	if (person.firstName)
+		return `${person.firstName.charAt(0)}${person.lastName.charAt(0)}`.toUpperCase();
+	return person.lastName.substring(0, 2).toUpperCase();
 }
 </script>
 
@@ -78,7 +81,7 @@ function getInitials(person: Correspondent): string {
 					{getInitials(person)}
 				</span>
 				<!-- Svelte auto-escapes — do not use {@html} here. -->
-				{person.lastName}, {person.firstName}
+				{person.displayName}
 			</div>
 		{/each}
 	{/if}