feat(coderd): add new dispatch logic for coder inbox (#16764)

This PR is [resolving the dispatch part of Coder
Inbocx](https://github.com/coder/internal/issues/403).

Since the DB layer has been merged - we now want to insert notifications
into Coder Inbox in parallel of the other delivery target.

To do so, we push two messages instead of one using the `Enqueue`
method.

coderd/notifications_test.go

@@ -296,6 +296,9 @@ func TestNotificationDispatchMethods(t *testing.T) {
 
 	var allMethods []string
 	for _, nm := range database.AllNotificationMethodValues() {
+		if nm == database.NotificationMethodInbox {
+			continue
+		}
 		allMethods = append(allMethods, string(nm))
 	}
 	slices.Sort(allMethods)