docs: add POLL_INTERVAL to README, .env.sample, and .env.example

- Document background polling and on-demand mode in README
- Add POLL_INTERVAL to all config examples (Docker, Compose, .env)
- New 'Background Polling' section in Features
- Sanitize leaked credentials in .env.example

.env.sample

@@ -14,6 +14,14 @@ PORT=3001
 # - silent: No logging
 LOG_LEVEL=info
 
+# Background polling interval in milliseconds (default: 5000)
+# sofarr polls all services in the background and caches results so
+# dashboard requests are near-instant.
+# Set to 0, "off", "false", or "disabled" to disable background polling.
+# When disabled, data is fetched on-demand when a user opens the dashboard
+# and cached for 30 seconds so other users benefit from the same fetch.
+# POLL_INTERVAL=5000
+
 # =============================================================================
 # EMBY (Authentication - Required)
 # =============================================================================
@@ -74,4 +82,5 @@ RADARR_INSTANCES=[{"name":"main","url":"https://radarr.example.com","apiKey":"yo
 # 3. URLs should include protocol (http:// or https://)
 # 4. For qBittorrent, ensure Web UI is enabled in settings
 # 5. User downloads are matched by tags in Sonarr/Radarr - tag your media!
+# 6. Background polling keeps data fresh; disable it for low-resource setups
 # =============================================================================