Targets section: one Add, then a type and Next, then its fields (closes #370)

The webhook page's targets section lists only its targets until Add is
clicked. Add shows a choice of target type with Next; Next shows only
that type's fields, with Save and Cancel. The `database` and `log`
types have no URL field, and the `slack` form gains max retries.

A refused target now shows the webhook page again with the form open on
its type, the values entered and the reason, instead of a bare text
page. Target validation returns that message rather than writing the
response; newTarget validates a whole new target for reuse by the
new-webhook page. The edit page still answers a refusal in plain text.

Model: opus-5-5

internal/handlers/export_test.go

@@ -143,22 +143,20 @@ func (s *Handlers) RenderTemplateForTest(
 // BuildSlackTargetConfigForTest exposes
 // buildSlackTargetConfig for use in the handlers_test package.
 func (s *Handlers) BuildSlackTargetConfigForTest(
-	w http.ResponseWriter,
-	r *http.Request,
+	ctx context.Context,
 	targetURL string,
-) (string, error) {
-	return s.buildSlackTargetConfig(w, r, targetURL)
+) (string, string) {
+	return s.buildSlackTargetConfig(ctx, targetURL)
 }
 
 // BuildHTTPTargetConfigForTest exposes buildHTTPTargetConfig
 // for use in the handlers_test package, taking the form fields
 // an HTTP target's configuration is built from.
 func (s *Handlers) BuildHTTPTargetConfigForTest(
-	w http.ResponseWriter,
-	r *http.Request,
+	ctx context.Context,
 	targetURL, headers, timeout string,
-) (string, error) {
-	return s.buildHTTPTargetConfig(w, r, targetFormInput{
+) (string, string) {
+	return s.buildHTTPTargetConfig(ctx, targetFormInput{
 		URL:     targetURL,
 		Headers: headers,
 		Timeout: timeout,
@@ -168,9 +166,6 @@ func (s *Handlers) BuildHTTPTargetConfigForTest(
 // BuildDatabaseTargetConfigForTest exposes
 // buildDatabaseTargetConfig for use in the handlers_test
 // package.
-func (s *Handlers) BuildDatabaseTargetConfigForTest(
-	w http.ResponseWriter,
-	expiry string,
-) (string, error) {
-	return s.buildDatabaseTargetConfig(w, newRequestForTest(), expiry)
+func BuildDatabaseTargetConfigForTest(expiry string) (string, string) {
+	return buildDatabaseTargetConfig(expiry)
 }