Validate Slack target URLs at creation time (closes #68) (#73)

Slack delivery targets were only checked by the request-time dialer guard, not at creation, giving them a weaker SSRF gate than HTTP targets.

This validates the Slack incoming-webhook URL with `delivery.ValidateTargetURL` in the Slack target creation path (`buildSlackTargetConfig`), before persisting, mirroring the existing HTTP-target path. On failure the create is rejected with the same clear, non-leaking user-facing error the HTTP path uses.

Adds handlers-package tests covering both an accepted public URL and a rejected private/reserved URL. Confined to `internal/handlers/`; `internal/delivery/` is unchanged.

Closes #68

Co-authored-by: sneak <sneak@sneak.berlin>
Co-authored-by: Jeffrey Paul <sneak@noreply.example.org>
Reviewed-on: #73
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/handlers/export_test.go

@@ -12,3 +12,13 @@ func (s *Handlers) RenderTemplateForTest(
 ) {
 	s.renderTemplate(w, r, pageTemplate, data)
 }
+
+// BuildSlackTargetConfigForTest exposes buildSlackTargetConfig
+// for use in the handlers_test package.
+func (s *Handlers) BuildSlackTargetConfigForTest(
+	w http.ResponseWriter,
+	r *http.Request,
+	targetURL string,
+) (string, error) {
+	return s.buildSlackTargetConfig(w, r, targetURL)
+}