New-webhook page: optional HTTP target URL and archive with pruning (closes #373)

The new-webhook page keeps name, description and retention and adds an
HTTP target URL field and an archive checkbox. A filled-in URL creates
an http target named HTTP; a checked archive shows a pruning choice
(never, 1h, 12h, 24h, 30d, 90d, 365d) and creates a database target
named Archive with that expiry. Each target goes through the add
target form's validation, and the webhook, its entrypoint and its
targets are created in one transaction. A refused form comes back with
every value entered, retention included.

Model: opus-5-5

internal/handlers/ui_copy_test.go

@@ -196,8 +196,6 @@ func TestCreateFormRetentionCopyMatchesBehaviour(t *testing.T) {
 	t.Cleanup(app.RequireStop)
 
 	body := renderPage(t, h, sess, "sources_new.html", map[string]any{
-		"Name":                 "",
-		"Description":          "",
 		"DefaultRetentionDays": database.DefaultRetentionDays,
 		dataKeyError:           "",
 	})