Factor shared scanner logic into scanners/common

The five scanners (ARP, SNMP, mDNS, SSDP, DHCP) duplicated their
persist-and-notify pipeline, device enrichment, and status state
machines across same-family files. Extract the shared logic so a change
lands in one place instead of three to five.

- scanners/common/pipeline.rs: single record_sighting() persist+notify
  path, replacing the per-scanner match blocks. ARP/SNMP now use the
  same merge rules as the passive scanners (keep a stored hostname,
  never overwrite a known IP with an empty one).
- scanners/common/enrichment.rs: build_device() for vendor/device-type
  lookup with the privacy-MAC service fallback.
- scanners/common/{active,passive}_status.rs: the two status state
  machines plus their tests, written once. Each scanner status.rs is now
  a thin wrapper over its own static.
- utils/network::format_mac(): replaces three identical copies.
- web_server/scanner_status.rs: Active/Passive response types and two
  handler helpers, replacing five near-identical structs+handlers. JSON
  field names are unchanged so the frontend is unaffected; only OpenAPI
  schema names change.

27 files changed, ~900 lines net removed. Build, clippy and all 113
tests (4 new) pass.

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

backend/src/web_server.rs

@@ -5,12 +5,10 @@ use crate::model::device_events::{DeviceEvent, DeviceEventScanner, DeviceEventTy
 use crate::model::devices::{Device, DeviceSummary};
 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::snmp_scanner::SnmpScannerStatusResponse;
-use crate::web_server::ssdp_scanner::SsdpScannerStatusResponse;
+use crate::web_server::scanner_status::{
+    ActiveScannerStatusResponse, PassiveScannerStatusResponse,
+};
 use axum::Json;
 use axum::extract::Request;
 use axum::http::StatusCode;
@@ -33,6 +31,7 @@ pub mod devices;
 pub mod dhcp_scanner;
 pub mod mdns_scanner;
 pub mod notifications;
+pub mod scanner_status;
 pub mod snmp_scanner;
 pub mod ssdp_scanner;
 pub mod utils;
@@ -74,11 +73,8 @@ pub mod utils;
         DeviceEvent,
         DeviceEventType,
         DeviceEventScanner,
-        ArpScannerStatusResponse,
-        MdnsScannerStatusResponse,
-        SsdpScannerStatusResponse,
-        DhcpScannerStatusResponse,
-        SnmpScannerStatusResponse,
+        ActiveScannerStatusResponse,
+        PassiveScannerStatusResponse,
     )),
     modifiers(&SecurityAddon),
     tags(