Show the 10 most recent deployments on the deployments page (closes #238)

An app's deployments page listed up to 50 deployments; it now lists the
10 most recent, newest first. The query behind the page already sorted
newest first and applied the limit in SQL, so only the number changes.
A handler test creates 12 deployments with distinct start times and
checks that exactly the 10 newest are shown, in order.

Model: opus-5-5

internal/handlers/app.go

@@ -28,7 +28,7 @@ const (
 	// recentDeploymentsLimit is the number of recent deployments to show.
 	recentDeploymentsLimit = 5
 	// deploymentsHistoryLimit is the number of deployments to show in history.
-	deploymentsHistoryLimit = 50
+	deploymentsHistoryLimit = 10
 )
 
 // redirectToApp issues a SeeOther redirect to the page for the given