docs(journeyitemcard): document why item.document! non-null assertion is safe

JourneyReader filters items to only those where document != null before
passing them here — the ! assertion is valid by caller invariant.

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

frontend/src/lib/geschichte/JourneyItemCard.svelte

@@ -11,6 +11,7 @@ interface Props {
 
 let { item }: Props = $props();
 
+// Safe: JourneyReader filters out items where document === null before rendering this component.
 const doc = $derived(item.document!);
 const formattedDate = $derived(doc.documentDate ? formatDate(doc.documentDate, 'short') : null);
 const ariaLabel = $derived(