Deduce vendor from mDNS services for privacy MACs

Modern devices increasingly use randomized ("Private WiFi Address") MACs,
which are locally administered and have no real OUI, so the MAC-prefix
vendor lookup returns nothing. Such devices still advertise distinctive
mDNS service types, which we now use to deduce their vendor.

- Extend the mDNS parser to also capture PTR service types alongside
  A-record hostnames.
- Add a service-type -> vendor mapping (data/mdns-service-vendor.json) and
  a service_vendor_finder, using canonical vendor names so device-type
  resolution still chains.
- Add utils::network::is_locally_administered to detect randomized MACs.
- In the mDNS scanner, fall back to service-based deduction only when the
  OUI lookup is empty and the MAC is locally administered.
- Group the three finders under a new data module.
- Preserve a known vendor (and its derived device_type) when a later
  sighting cannot deduce one, and suppress the spurious "vendor changed"
  notification in that case.

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

backend/src/data/mac_vendor_finder.rs

@@ -0,0 +1,53 @@
+use lazy_static::lazy_static;
+use log::{debug, error, info};
+use serde::Deserialize;
+use std::collections::HashMap;
+
+#[derive(Deserialize)]
+#[serde(rename_all = "camelCase")]
+struct MacRecord {
+    pub mac_prefix: String,
+    pub vendor_name: String,
+    // private: bool,
+    // block_type: String,
+    // last_update: String,
+}
+
+// Initialize mac vendors database as a static lazy loaded unit
+lazy_static! {
+    static ref MAC_VENDORS_DATABASE: HashMap<String, String> = {
+        info!("Loading MAC vendor database into memory");
+
+        let mut database = HashMap::new();
+        let data = include_str!("../../data/mac-vendors-export.json");
+
+        let json: Vec<MacRecord> = match serde_json::from_str(data) {
+            Ok(value) => value,
+            Err(error) => {
+                error!(
+                    "Error parsing mac vendors database (data/mac-vendors-export.json): {error}"
+                );
+                panic!(
+                    "Error parsing mac vendors database (data/mac-vendors-export.json): {error}"
+                );
+            }
+        };
+
+        debug!("Found {} records in the database", json.len());
+
+        for el in json {
+            database.insert(el.mac_prefix.to_uppercase(), el.vendor_name);
+        }
+
+        info!("MAC vendor database loaded");
+        database
+    };
+}
+
+// Find a vendor based on the MAC prefix
+pub fn find(mac_prefix: String) -> String {
+    MAC_VENDORS_DATABASE
+        .get(&mac_prefix.to_uppercase())
+        .unwrap_or(&"".to_string())
+        .to_string()
+}