Add ARP scanner status API endpoint and rename scanner module

Renames scanner.rs to arp_scanner.rs to future-proof for additional scanner
types. Renames the [timings] config section to [arp_scanner] and simplifies
field names (arp_sender_timeout -> sender_timeout, arp_scan_duration ->
scan_duration). Introduces arp_scanner_status module to track whether the
scan is actively running or sleeping, and exposes this via a new
GET /api/arp_scanner/status endpoint returning is_running,
running_for_seconds, and next_run_in_seconds.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/settings.rs

@@ -22,10 +22,10 @@ pub struct Log {
 }
 
 #[derive(Debug, Deserialize, Clone)]
-pub struct Timings {
+pub struct ArpScanner {
     pub wait_between_scans: DurationString,
-    pub arp_sender_timeout: DurationString,
-    pub arp_scan_duration: DurationString,
+    pub sender_timeout: DurationString,
+    pub scan_duration: DurationString,
 }
 
 #[derive(Debug, Deserialize, Clone)]
@@ -66,7 +66,7 @@ pub struct Settings {
     pub database: Database,
     pub networking: Networking,
     pub log: Log,
-    pub timings: Timings,
+    pub arp_scanner: ArpScanner,
     pub notifications: Notifications,
     pub web_server: WebServer,
     #[serde(default)]