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/routes/documents/[id]/+page.server.ts

@@ -1,6 +1,7 @@
 import { error, redirect } from '@sveltejs/kit';
 import { createApiClient } from '$lib/api.server';
 import { getErrorMessage } from '$lib/errors';
+import { inferredRelationshipLabel } from '$lib/relationshipLabels';
 
 export async function load({ params, fetch }) {
 	const { id } = params;
@@ -15,5 +16,38 @@ export async function load({ params, fetch }) {
 		throw error(docResult.response.status, getErrorMessage(code));
 	}
 
-	return { document: docResult.data! };
+	const document = docResult.data!;
+	const inferredRelationship = await loadInferredRelationship(api, document);
+
+	return { document, inferredRelationship };
+}
+
+async function loadInferredRelationship(
+	api: ReturnType<typeof createApiClient>,
+	document: {
+		sender?: { id: string; familyMember?: boolean } | null;
+		receivers?: { id: string; familyMember?: boolean }[];
+	}
+): Promise<{ labelFromA: string; labelFromB: string } | null> {
+	const sender = document.sender;
+	const receivers = document.receivers ?? [];
+
+	// The badge is shown only when both endpoints are family members and the
+	// document has exactly one receiver.
+	if (!sender?.familyMember) return null;
+	if (receivers.length !== 1) return null;
+	const receiver = receivers[0];
+	if (!receiver?.familyMember) return null;
+
+	const result = await api.GET('/api/persons/{aId}/relationship-to/{bId}', {
+		params: { path: { aId: sender.id, bId: receiver.id } }
+	});
+
+	if (result.response.status === 404) return null;
+	if (!result.response.ok || !result.data) return null;
+
+	return {
+		labelFromA: inferredRelationshipLabel(result.data.labelFromA),
+		labelFromB: inferredRelationshipLabel(result.data.labelFromB)
+	};
 }

frontend/src/routes/documents/[id]/+page.svelte

@@ -395,6 +395,7 @@ onMount(() => {
 		canWrite={canWrite}
 		fileUrl={fileLoader.fileUrl}
 		bind:transcribeMode={transcribeMode}
+		inferredRelationship={data.inferredRelationship}
 	/>
 
 	<div class="relative flex-1 overflow-hidden {transcribeMode ? 'flex flex-col md:flex-row' : ''}">