Add passive mDNS/Bonjour discovery scanner

Introduce a second discovery module that passively listens for mDNS
multicast announcements (224.0.0.251:5353) and feeds discovered devices
into the existing devices/events/notifications pipeline, running in its
own task alongside the ARP scanner.

Since mDNS carries an IP and hostname but no MAC (the device key), the
module resolves IP->MAC via the OS ARP cache with a targeted ARP-probe
fallback. The advertised hostname is stored in a new optional `name`
column on devices (blank for ARP-only devices); the single `update`
writes `name` only when set so ARP rescans never clobber it. Device
names are included in event/notification messages.

Adds a GET /api/mdns_scanner/status endpoint (is_listening, devices_seen,
last_device_seen_seconds_ago) wired into OpenAPI, an optional
[mdns_scanner] config section, and the corresponding Nix module option.
Also aligns the Nix module's stale `timings` section with the current
`arp_scanner` schema.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/settings.rs

@@ -28,6 +28,19 @@ pub struct ArpScanner {
     pub scan_duration: DurationString,
 }
 
+#[derive(Debug, Deserialize, Clone)]
+pub struct MdnsScanner {
+    pub probe_timeout: DurationString,
+}
+
+impl Default for MdnsScanner {
+    fn default() -> Self {
+        MdnsScanner {
+            probe_timeout: DurationString::try_from("2s".to_string()).unwrap(),
+        }
+    }
+}
+
 #[derive(Debug, Deserialize, Clone)]
 pub struct Pushover {
     pub token: String,
@@ -71,6 +84,8 @@ pub struct Settings {
     pub web_server: WebServer,
     #[serde(default)]
     pub retention: Retention,
+    #[serde(default)]
+    pub mdns_scanner: MdnsScanner,
 }
 // End configuration structure
 // -----------------------------------------------------------