Fix all clippy warnings in the Rust backend

Addresses all 31 warnings emitted by cargo clippy: redundant field
names in struct initialisers, unnecessary unwrap after is_some checks
(replaced with if-let), needless borrows, filter+next replaced with
find, iter().count() replaced with len(), clone on Copy type, and
manual match-to-Option replaced with .ok().

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

backend/src/model/devices.rs

@@ -25,10 +25,10 @@ impl Device {
         last_seen: DateTime<Utc>,
     ) -> Self {
         Self {
-            mac_address: mac_address,
-            ipv4_address: ipv4_address,
-            vendor: vendor,
-            last_seen: last_seen,
+            mac_address,
+            ipv4_address,
+            vendor,
+            last_seen,
             is_registered: false,
             owner: "".to_string(),
             device_type: "".to_string(),