perf: batch-load app history to eliminate N+1 fetches on board load

Previously each AppWidget fetched /api/apps/{id}/history individually
on mount, causing N sequential HTTP requests. Now the board page
server load fetches all app histories in a single Prisma query via
getBatchStatusHistory() and passes them to AppWidget via Svelte
context. AppWidget uses the pre-loaded data immediately with a
fallback fetch for non-board contexts.

src/routes/boards/[boardId]/+page.server.ts

@@ -75,7 +75,26 @@ export const load: PageServerLoad = async ({ params, locals }) => {
 			}));
 		}
 
-		return { board, canEdit, allApps, users, groups };
+		// Batch-load sparkline history for all apps on this board (single query)
+		const appIdsOnBoard = board.sections
+			.flatMap((s: { widgets: { appId: string | null }[] }) => s.widgets)
+			.map((w: { appId: string | null }) => w.appId)
+			.filter((id: string | null): id is string => id !== null);
+
+		const historyMap = appIdsOnBoard.length > 0
+			? await appService.getBatchStatusHistory(appIdsOnBoard)
+			: new Map();
+
+		// Serialize the Map to a plain object for the client
+		const appHistories: Record<string, { history: { status: string; responseTime: number | null; checkedAt: string }[]; uptimePercent: number }> = {};
+		for (const [appId, data] of historyMap) {
+			appHistories[appId] = {
+				history: data.history.map((h) => ({ ...h, checkedAt: h.checkedAt.toISOString() })),
+				uptimePercent: data.uptimePercent
+			};
+		}
+
+		return { board, canEdit, allApps, users, groups, appHistories };
 	} catch (err) {
 		const message = err instanceof Error ? err.message : 'Board not found';
 		if (message.includes('not found')) {