fix(documents): surface timeline density fetch failures via console.warn (#385)

Previously a 5xx, network blip, or JSON parse error all collapsed
into the same silent "no buckets" rendering. The widget still
degrades gracefully — failure should not block the document list —
but operators and Sentry now see the failure in browser devtools
instead of having to reverse-engineer a missing chart.

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

frontend/src/lib/document/timeline.ts

@@ -208,14 +208,18 @@ export async function fetchDensity(
 
 	try {
 		const response = await fetch(buildDensityUrl(filters));
-		if (!response.ok) return EMPTY;
+		if (!response.ok) {
+			console.warn(`[timeline] density fetch responded with ${response.status}`);
+			return EMPTY;
+		}
 		const body = (await response.json()) as DocumentDensityResult;
 		return {
 			density: body.buckets,
 			minDate: body.minDate ?? null,
 			maxDate: body.maxDate ?? null
 		};
-	} catch {
+	} catch (error) {
+		console.warn('[timeline] density fetch failed', error);
 		return EMPTY;
 	}
 }