Display unknown values as a dash and centralise repeated literals

Backend: notifications now show a plain "-" for an absent name, vendor, or
device type (was empty string / "(unknown)" / "Unknown"), via a single
UNKNOWN_PLACEHOLDER constant.

Frontend:
- Empty/unknown values render as an em dash everywhere, centralised in a new
  Placeholders.emptyValue constant (replaces inline '—' and '(unknown)').
- Route paths moved to a new Routes class, used by the router and every
  navigation call site.
- Device event type modelled as a DeviceEventType enum mirroring the backend
  (NewDevice/DeviceSeen) instead of bare string comparisons.
- Hardcoded EdgeInsets/SizedBox spacing replaced with existing Insets tokens.

Tests and formatting updated; all backend and frontend tests pass.

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

frontend/lib/model/device_event.dart

@@ -1,8 +1,10 @@
+import 'device_event_type.dart';
+
 class DeviceEvent {
   final int id;
   final String macAddress;
   final DateTime createdOn;
-  final String eventType;
+  final DeviceEventType eventType;
   final String ipv4Address;
   final String vendor;
   final String scanner;
@@ -22,7 +24,7 @@ class DeviceEvent {
       id: json['id'] as int,
       macAddress: json['mac_address'] as String,
       createdOn: DateTime.parse(json['created_on'] as String),
-      eventType: json['event_type'] as String,
+      eventType: DeviceEventType.fromString(json['event_type'] as String),
       ipv4Address: json['ipv4_address'] as String,
       vendor: json['vendor'] as String,
       scanner: json['scanner'] as String,