api/v2: add user get and list (Tailscale compat)

Backs the tailscale_user and tailscale_users data sources. getUser maps
gorm/ErrUserNotFound to 404; unmodelled fields are honest constants,
deviceCount/lastSeen/currentlyConnected aggregate the user's nodes.

hscontrol/api/v2/api.go

@@ -244,6 +244,9 @@ const (
 
 	ScopeFeatureSettings     Scope = "feature_settings"
 	ScopeFeatureSettingsRead Scope = "feature_settings:read"
+
+	ScopeUsers     Scope = "users"
+	ScopeUsersRead Scope = "users:read"
 )
 
 // scopeMetaKey keys the per-operation required Scope in huma.Operation.Metadata.