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/DocumentTopBar.svelte

@@ -30,9 +30,16 @@ type Props = {
 	canWrite: boolean;
 	fileUrl: string;
 	transcribeMode: boolean;
+	inferredRelationship?: { labelFromA: string; labelFromB: string } | null;
 };
 
-let { doc, canWrite, fileUrl, transcribeMode = $bindable() }: Props = $props();
+let {
+	doc,
+	canWrite,
+	fileUrl,
+	transcribeMode = $bindable(),
+	inferredRelationship = null
+}: Props = $props();
 
 let detailsOpen = $state(false);
 
@@ -275,6 +282,7 @@ let mobileMenuOpen = $state(false);
 				sender={doc.sender ?? null}
 				receivers={doc.receivers ? [...doc.receivers] : []}
 				tags={doc.tags ? [...doc.tags] : []}
+				inferredRelationship={inferredRelationship}
 			/>
 		</div>
 	{/if}