fix(coderd): workspaceapps: update last_used_at when workspace app reports stats (#11603)

- Adds a new query BatchUpdateLastUsedAt
- Adds calls to BatchUpdateLastUsedAt in app stats handler upon flush
- Passes a stats flush channel to apptest setup scaffolding and updates unit tests to assert modifications to LastUsedAt.

coderd/workspaceapps/apptest/setup.go

@@ -71,6 +71,7 @@ type Deployment struct {
 	SDKClient      *codersdk.Client
 	FirstUser      codersdk.CreateFirstUserResponse
 	PathAppBaseURL *url.URL
+	FlushStats     func()
 }
 
 // DeploymentFactory generates a deployment with an API client, a path base URL,