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/arp/scanner.rs

@@ -9,6 +9,11 @@ use log::{debug, info};
 use tokio::time::{Duration, sleep};
 
 pub async fn scan() -> Result<(), Box<dyn std::error::Error>> {
+    if !get_settings().arp_scanner.enabled {
+        info!("ARP scanner disabled in configuration; not starting");
+        return Ok(());
+    }
+
     loop {
         status::set_running();