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.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' : ''}">