fix: ensure targets are propagated to inbox (#16985)

Currently the `targets` column in `inbox_notifications` doesn't get
filled. This PR fixes that. Rather than give targets special treatment,
we should put it in the payload like everything else. This correctly
propagates notification targets to the inbox table without much code
change.

coderd/notifications/notifications_test.go

@@ -1333,7 +1333,6 @@ func TestNotificationTemplates_Golden(t *testing.T) {
 				)
 				require.NoError(t, err)
 
-				tc.payload.Targets = append(tc.payload.Targets, user.ID)
 				_, err = smtpEnqueuer.EnqueueWithData(
 					ctx,
 					user.ID,
@@ -1466,7 +1465,7 @@ func TestNotificationTemplates_Golden(t *testing.T) {
 					tc.payload.Labels,
 					tc.payload.Data,
 					user.Username,
-					user.ID,
+					tc.payload.Targets...,
 				)
 				require.NoError(t, err)