Add per-scanner enable/disable configuration

Each scanner (ARP, mDNS, SSDP/UPnP, DHCP) can now be turned off via an
`enabled` flag in its config section, defaulting to true so existing
deployments are unchanged. A disabled scanner's entry function returns
early and never starts.

Documented in the README options table (also fixing the stale `timings.*`
key names to the actual `arp_scanner.*` keys), and added to the TOML
samples and the NixOS module.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/scanners/ssdp/scanner.rs

@@ -17,6 +17,11 @@ use crate::settings::get_settings;
 /// Passively listen for SSDP/UPnP NOTIFY announcements and feed discovered devices into the same
 /// pipeline used by the ARP and mDNS scanners (devices table + events + notifications).
 pub async fn listen() -> Result<(), Box<dyn std::error::Error>> {
+    if !get_settings().ssdp_scanner.enabled {
+        info!("SSDP scanner disabled in configuration; not starting");
+        return Ok(());
+    }
+
     let interface = get_settings().networking.interface.clone();
     let socket = finder::open_socket(interface.clone())?;
     status::set_listening();