Add passive DHCP-snooping scanner

Listen for DHCP DISCOVER/REQUEST broadcasts on UDP 67 to catch devices as
early as possible — a device must request an address before doing almost
anything else, often before it has an IP.

Follows the mDNS/SSDP scanner pattern (finder/scanner/status modules) and
feeds the shared devices/events/notifications pipeline. The client MAC is
taken directly from the packet's chaddr, so no ARP probe is needed; a
DISCOVER with no assigned IP reuses any previously recorded address rather
than clobbering it. Exposes GET /api/dhcp_scanner/status, wired into the
OpenAPI generation, and adds a Dhcp variant to DeviceEventScanner.

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

backend/src/web_server.rs

@@ -6,6 +6,7 @@ use crate::model::notifications::{Notification, NotificationType};
 use crate::settings::get_settings;
 use crate::web_server::arp_scanner::ArpScannerStatusResponse;
 use crate::web_server::devices::{RegisterDevicePayload, UpdateDevicePayload};
+use crate::web_server::dhcp_scanner::DhcpScannerStatusResponse;
 use crate::web_server::mdns_scanner::MdnsScannerStatusResponse;
 use crate::web_server::ssdp_scanner::SsdpScannerStatusResponse;
 use axum::Json;
@@ -27,6 +28,7 @@ use utoipa_swagger_ui::SwaggerUi;
 pub mod arp_scanner;
 pub mod device_events;
 pub mod devices;
+pub mod dhcp_scanner;
 pub mod mdns_scanner;
 pub mod notifications;
 pub mod ssdp_scanner;
@@ -56,6 +58,7 @@ pub mod utils;
         arp_scanner::status,
         mdns_scanner::status,
         ssdp_scanner::status,
+        dhcp_scanner::status,
     ),
     components(schemas(
         Device,
@@ -70,6 +73,7 @@ pub mod utils;
         ArpScannerStatusResponse,
         MdnsScannerStatusResponse,
         SsdpScannerStatusResponse,
+        DhcpScannerStatusResponse,
     )),
     modifiers(&SecurityAddon),
     tags(
@@ -79,6 +83,7 @@ pub mod utils;
         (name = "arp_scanner", description = "ARP scanner process status"),
         (name = "mdns_scanner", description = "mDNS/Bonjour scanner process status"),
         (name = "ssdp_scanner", description = "SSDP/UPnP scanner process status"),
+        (name = "dhcp_scanner", description = "DHCP scanner process status"),
     )
 )]
 struct ApiDoc;
@@ -125,6 +130,7 @@ pub async fn serve() -> Result<(), Box<dyn Error>> {
         .route("/api/arp_scanner/status", get(arp_scanner::status))
         .route("/api/mdns_scanner/status", get(mdns_scanner::status))
         .route("/api/ssdp_scanner/status", get(ssdp_scanner::status))
+        .route("/api/dhcp_scanner/status", get(dhcp_scanner::status))
         .route("/api/notifications", get(notifications::list))
         .route(
             "/api/notifications/mark_all_as_old",