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.example

@@ -1,5 +1,10 @@
 # Server Configuration
 PORT=3001
+LOG_LEVEL=info
+
+# Background polling interval in ms (default: 5000)
+# Set to 0 or "off" to disable and fetch on-demand instead
+# POLL_INTERVAL=5000
 
 # Emby Configuration (single instance)
 EMBY_URL=http://localhost:8096
@@ -16,7 +21,4 @@ SONARR_INSTANCES=[{"name": "Primary", "url": "http://localhost:8989", "apiKey":
 RADARR_INSTANCES=[{"name": "Primary", "url": "http://localhost:7878", "apiKey": "your_api_key"}]
 
 # qBittorrent Instances (JSON array)
-QBITTORRENT_INSTANCES=[
-  {"name": "ransackedcrew", "url": "https://qbittorrent.ransackedcrew.info", "username": "gronod", "password": "K32D&JDjtHA&mC"},
-  {"name": "i3omb", "url": "https://qbittorrent.i3omb.com", "username": "admin", "password": "b053288369XX!"}
-]
+QBITTORRENT_INSTANCES=[{"name": "main", "url": "http://localhost:8080", "username": "admin", "password": "your_password"}]