perf: persist qBittorrent clients between requests

- Reuse client instances so auth cookies survive across requests
- Eliminates redundant login round-trips on every dashboard refresh
- Clients still re-authenticate automatically if session expires (403)

server/utils/qbittorrent.js

@@ -96,14 +96,19 @@ class QBittorrentClient {
   }
 }
 
+// Persist clients so auth cookies survive between requests
+let persistedClients = null;
+
 function getClients() {
+  if (persistedClients) return persistedClients;
   const instances = getQbittorrentInstances();
   if (instances.length === 0) {
     logToFile('[qBittorrent] No instances configured');
     return [];
   }
-  logToFile(`[qBittorrent] Created ${instances.length} client(s)`);
-  return instances.map(inst => new QBittorrentClient(inst));
+  logToFile(`[qBittorrent] Created ${instances.length} persistent client(s)`);
+  persistedClients = instances.map(inst => new QBittorrentClient(inst));
+  return persistedClients;
 }
 
 async function getAllTorrents() {