Set SNMP scanner default timings to 10m/5s

A typical router keeps active devices' ARP entries fresh continuously,
so a 10-minute poll interval stays well within common ARP cache TTLs
while keeping device-event churn modest (each poll records a DeviceSeen
event per device). A 5s per-request timeout adds margin for a busy
agent or large ARP table at no cost on the happy path.

Updates the default in settings.rs and the sample TOML, README and
NixOS module. TODO timing-review item narrowed to the ARP scanner.

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

backend/src/settings.rs

@@ -98,8 +98,8 @@ impl Default for SnmpScanner {
             enabled: false,
             target: String::new(),
             community: String::new(),
-            wait_between_scans: DurationString::try_from("2m".to_string()).unwrap(),
-            timeout: DurationString::try_from("3s".to_string()).unwrap(),
+            wait_between_scans: DurationString::try_from("10m".to_string()).unwrap(),
+            timeout: DurationString::try_from("5s".to_string()).unwrap(),
         }
     }
 }