refactor(person-mention): split fetchHoverData into pure load + cache wrapper

Felix #1: fetchHoverData was doing four things — cache lookup, fetch, JSON
parsing, 404 normalisation. Split into:

  loadHoverData(personId)       — pure fetch + 404→null + non-OK→throw
  getOrFetchHoverData(personId) — five-line cache wrapper around the above

Also document the cache-lifetime trade-off (Markus #4, Elicit OQ-372-02):
the cache is per-mount, so closing and reopening the transcription panel
rebuilds it. That's intentional given the read-only nature of the view —
revisit if stale-card user reports surface.

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

frontend/src/lib/components/TranscriptionReadView.svelte

@@ -26,9 +26,14 @@ let { blocks, onParagraphClick, highlightBlockId = null }: Props = $props();
 
 let sorted = $derived([...blocks].sort((a, b) => a.sortOrder - b.sortOrder));
 
-// Per-page in-memory cache: a sweep across 20 mentions of the same person
+// Per-component (per-mount) in-memory cache: a sweep across 20 mentions of the
-// must not fire 20 backend calls (B15.5). The Promise<HoverData | null> shape
+// same person must not fire 20 backend calls (B15.5). The Promise<HoverData | null>
-// lets simultaneous hovers share the same in-flight fetch.
+// shape lets simultaneous hovers share the same in-flight fetch.
+//
+// Trade-off: closing and re-opening the transcription panel rebuilds this cache
+// (Elicit OQ-372-02). That's intentional — staleness from another tab deleting
+// a person is rare in this read-only view, and a per-document/global cache would
+// complicate invalidation. If user reports on stale cards accumulate, revisit.
 const hoverCache = new SvelteMap<string, Promise<HoverData | null>>();
 const deletedPersonIds = new SvelteSet<string>();
 
@@ -57,25 +62,32 @@ function renderBlockHtml(block: TranscriptionBlockData): SafeHtml {
 		.join('') as SafeHtml;
 }
 
-function fetchHoverData(personId: string): Promise<HoverData | null> {
+/**
-	let cached = hoverCache.get(personId);
+ * Fetches person + relationships from the backend. 404 returns null
+ * (deleted person — caller marks the link as tombstoned). Any other
+ * non-OK response throws so the caller can render the error state.
+ */
+async function loadHoverData(personId: string): Promise<HoverData | null> {
+	const personRes = await fetch(`/api/persons/${personId}`);
+	if (personRes.status === 404) return null;
+	if (!personRes.ok) throw new Error(`person fetch failed: ${personRes.status}`);
+	const person = (await personRes.json()) as Person;
+
+	const relRes = await fetch(`/api/persons/${personId}/relationships`);
+	const relationships: RelationshipDTO[] = relRes.ok
+		? ((await relRes.json()) as RelationshipDTO[])
+		: [];
+	return { person, relationships };
+}
+
+/** Cache wrapper around `loadHoverData` — first hover fires the fetch, all
+ * subsequent hovers (and concurrent in-flight ones) share the same Promise. */
+function getOrFetchHoverData(personId: string): Promise<HoverData | null> {
+	const cached = hoverCache.get(personId);
 	if (cached) return cached;
-
+	const promise = loadHoverData(personId);
-	cached = (async () => {
+	hoverCache.set(personId, promise);
-		const personRes = await fetch(`/api/persons/${personId}`);
+	return promise;
-		if (personRes.status === 404) return null;
-		if (!personRes.ok) throw new Error(`person fetch failed: ${personRes.status}`);
-		const person = (await personRes.json()) as Person;
-
-		const relRes = await fetch(`/api/persons/${personId}/relationships`);
-		const relationships: RelationshipDTO[] = relRes.ok
-			? ((await relRes.json()) as RelationshipDTO[])
-			: [];
-		return { person, relationships };
-	})();
-
-	hoverCache.set(personId, cached);
-	return cached;
 }
 
 function currentViewport() {
@@ -102,7 +114,7 @@ async function handleMentionEnter(event: Event) {
 	activeCard = { personId, cardId, position, state: { status: 'loading' } };
 
 	try {
-		const data = await fetchHoverData(personId);
+		const data = await getOrFetchHoverData(personId);
 		// Bail if a different mention is now active
 		if (!activeCard || activeCard.personId !== personId) return;