add admin user last used support

backend/internal/handler/dto/mappers.go

@@ -68,6 +68,7 @@ func UserFromServiceAdmin(u *service.User) *AdminUser {
 	return &AdminUser{
 		User:       *base,
 		Notes:      u.Notes,
+		LastUsedAt: u.LastUsedAt,
 		GroupRates: u.GroupRates,
 	}
 }

backend/internal/handler/dto/types.go

@@ -36,7 +36,8 @@ type User struct {
 type AdminUser struct {
 	User
 
-	Notes string `json:"notes"`
+	Notes      string     `json:"notes"`
+	LastUsedAt *time.Time `json:"last_used_at"`
 	// GroupRates 用户专属分组倍率配置
 	// map[groupID]rateMultiplier
 	GroupRates map[int64]float64 `json:"group_rates,omitempty"`

backend/internal/handler/dto/user_mapper_activity_test.go

@@ -13,6 +13,7 @@ func TestUserFromServiceAdmin_MapsActivityTimestamps(t *testing.T) {
 
 	lastLoginAt := time.Date(2026, time.April, 20, 10, 0, 0, 0, time.UTC)
 	lastActiveAt := lastLoginAt.Add(15 * time.Minute)
+	lastUsedAt := lastLoginAt.Add(45 * time.Minute)
 
 	out := UserFromServiceAdmin(&service.User{
 		ID:           42,
@@ -22,11 +23,14 @@ func TestUserFromServiceAdmin_MapsActivityTimestamps(t *testing.T) {
 		Status:       service.StatusActive,
 		LastLoginAt:  &lastLoginAt,
 		LastActiveAt: &lastActiveAt,
+		LastUsedAt:   &lastUsedAt,
 	})
 
 	require.NotNil(t, out)
 	require.NotNil(t, out.LastLoginAt)
 	require.NotNil(t, out.LastActiveAt)
+	require.NotNil(t, out.LastUsedAt)
 	require.WithinDuration(t, lastLoginAt, *out.LastLoginAt, time.Second)
 	require.WithinDuration(t, lastActiveAt, *out.LastActiveAt, time.Second)
+	require.WithinDuration(t, lastUsedAt, *out.LastUsedAt, time.Second)
 }