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/index.js

@@ -54,7 +54,7 @@ const radarrRoutes = require('./routes/radarr');
 const embyRoutes = require('./routes/emby');
 const dashboardRoutes = require('./routes/dashboard');
 const authRoutes = require('./routes/auth');
-const { startPoller, POLL_INTERVAL } = require('./utils/poller');
+const { startPoller, POLL_INTERVAL, POLLING_ENABLED } = require('./utils/poller');
 
 const app = express();
 const PORT = process.env.PORT || 3001;
@@ -80,7 +80,7 @@ app.listen(PORT, () => {
   console.log(`  sofarr - Your Downloads Dashboard`);
   console.log(`  Server running on port ${PORT}`);
   console.log(`  Log level: ${process.env.LOG_LEVEL || 'info'}`);
-  console.log(`  Poll interval: ${POLL_INTERVAL}ms`);
+  console.log(`  Polling: ${POLLING_ENABLED ? POLL_INTERVAL + 'ms' : 'disabled (on-demand)'}`);
   console.log(`=================================`);
   startPoller();
 });