api: Allow SubscriptionUpdateEvent with unrecognized property

Our event poll shouldn't crash on subscription/update events about
data that we don't store and act on. This bugfix allows servers to
add new Subscription fields and send corresponding events without
causing the app to discard its data and reregister a new event
queue.

There's one field we'd like to remove code for, in_home_view, which
was deprecated in FL 139 but is still sent (as of CZO on 2025-10-03)
alongside an event with the modern is_muted field. This change
prepares for that by gracefully ignoring
SubscriptionProperty.unknown, which will be how in_home_view gets
represented.

Author: chrisbobbe

lib/api/model/events.dart

@@ -762,6 +762,7 @@ class SubscriptionUpdateEvent extends SubscriptionEvent {
 
   final int streamId;
 
+  @JsonKey(unknownEnumValue: SubscriptionProperty.unknown)
   final SubscriptionProperty property;
 
   /// The new value, or null if we don't recognize the setting.