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>

examples/sample_oott.toml

@@ -8,16 +8,22 @@ path = "./oott.db" # Database path. Make sure it's writeable for the user runnin
 level = "info" # off, error, warn, info, debug, trace
 
 [arp_scanner]
+enabled=true # Set to false to disable the ARP scanner
 wait_between_scans="30m" # Wait time between scans. This does not include the scan time
 sender_timeout="1m" # If the ARP sender process takes longer than this it will be stopped (for a class C network - 254 IPs - it should take less than a minute)
 scan_duration="10m" # How long to wait for response packets on each scan (5m to 10m is a good timeframe for a class B or C network)
 
 [mdns_scanner]
+enabled=true # Set to false to disable the mDNS/Bonjour scanner
 probe_timeout="2s" # When an mDNS-discovered IP is not in the OS ARP cache, how long to wait for a targeted ARP probe reply to resolve its MAC address
 
 [ssdp_scanner]
+enabled=true # Set to false to disable the SSDP/UPnP scanner
 probe_timeout="2s" # When an SSDP/UPnP-discovered IP is not in the OS ARP cache, how long to wait for a targeted ARP probe reply to resolve its MAC address
 
+[dhcp_scanner]
+enabled=true # Set to false to disable the DHCP scanner
+
 [notifications]
 method="pushover" # For now just pushover, you can set this to "none" to avoid sending notifications (it will just log)
 notify_when_not_seen_for="1w" # Send a notification if a device comes back online after not being seen for this timeframe