feat(stammbaum): show inferred relationship in the document drawer

- New presentational RelationshipBadge component (labelFromA → arrow →
  labelFromB) wired into DocumentMetadataDrawer's Personen column,
  rendered after the receivers block when both endpoints are family
  members.
- DocumentTopBar gains an optional inferredRelationship prop and
  passes it through.
- documents/[id]/+page.server.ts loads the badge: only when sender is
  a family member, exactly one receiver, and that receiver is also a
  family member; 404 (no path) → null.
- relationshipLabels.ts maps the backend label keys (parent/child/...)
  to localised strings, so the server load returns badge-ready strings.

Refs #358.

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

frontend/src/lib/components/DocumentMetadataDrawer.svelte

@@ -3,6 +3,7 @@ import { m } from '$lib/paraglide/messages.js';
 import { formatDate } from '$lib/utils/date';
 import { formatDocumentStatus } from '$lib/utils/documentStatusLabel';
 import { getInitials, personAvatarColor } from '$lib/utils/personFormat';
+import RelationshipBadge from '$lib/components/RelationshipBadge.svelte';
 
 type Person = { id: string; firstName?: string | null; lastName: string; displayName: string };
 type Tag = { id: string; name: string };
@@ -14,9 +15,18 @@ type Props = {
 	sender: Person | null;
 	receivers: Person[];
 	tags: Tag[];
+	inferredRelationship?: { labelFromA: string; labelFromB: string } | null;
 };
 
-let { documentDate, location, status, sender, receivers, tags }: Props = $props();
+let {
+	documentDate,
+	location,
+	status,
+	sender,
+	receivers,
+	tags,
+	inferredRelationship = null
+}: Props = $props();
 
 const VISIBLE_RECEIVER_LIMIT = 5;
 
@@ -112,6 +122,12 @@ function getFullName(person: Person): string {
 							{/if}
 						</div>
 					{/if}
+					{#if inferredRelationship}
+						<RelationshipBadge
+							labelFromA={inferredRelationship.labelFromA}
+							labelFromB={inferredRelationship.labelFromB}
+						/>
+					{/if}
 				</div>
 			{:else}
 				<p class="font-serif text-sm text-ink-3">{m.doc_details_no_persons()}</p>