feat: make background polling disablable with on-demand fallback

- Set POLL_INTERVAL=0, off, false, or disabled to disable background polling
- When disabled, data is fetched on-demand when a user opens the dashboard
- On-demand results cached for 30s so other users benefit from fresh data
- A user with a faster refresh rate keeps the cache warm for everyone
- When polling is enabled, behaviour is unchanged (default 5s)

server/routes/dashboard.js

@@ -3,6 +3,7 @@ const router = express.Router();
 
 const { mapTorrentToDownload } = require('../utils/qbittorrent');
 const cache = require('../utils/cache');
+const { pollAllServices, POLLING_ENABLED } = require('../utils/poller');
 
 const EMBY_URL = process.env.EMBY_URL;
 const EMBY_API_KEY = process.env.EMBY_API_KEY;
@@ -105,7 +106,14 @@ router.get('/user-downloads', async (req, res) => {
     const showAll = isAdmin && req.query.showAll === 'true';
     console.log(`[Dashboard] Serving downloads for user: ${user.name} (${username}), isAdmin: ${isAdmin}, showAll: ${showAll}`);
 
-    // Read all data from poller cache
+    // When polling is disabled, fetch on-demand if cache has expired
+    // The fetched data is cached (30s TTL) so subsequent requests from any user reuse it
+    if (!POLLING_ENABLED && !cache.get('poll:sab-queue')) {
+      console.log(`[Dashboard] Cache expired and polling disabled, fetching on-demand...`);
+      await pollAllServices();
+    }
+
+    // Read all data from cache
     const sabQueueData = cache.get('poll:sab-queue') || { slots: [] };
     const sabHistoryData = cache.get('poll:sab-history') || { slots: [] };
     const sonarrTagsResults = cache.get('poll:sonarr-tags') || [];