fix: enable CI tests with build tags and add billing tests

- Add build tag to studio/embed.go to exclude from test builds
- Create stub embed_testing.go for test builds
- Update CI pipeline to use -tags testing
- Add tests for webhook signature verification and status normalization

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

studio/embed.go

@@ -1,3 +1,5 @@
+//go:build !testing
+
 package studio
 
 import (

studio/embed_testing.go

@@ -0,0 +1,18 @@
+//go:build testing
+
+package studio
+
+import (
+	"io/fs"
+	"net/http"
+)
+
+var distFS fs.FS
+
+func Handler() http.Handler {
+	return http.NotFoundHandler()
+}
+
+func Read(name string) ([]byte, error) {
+	return nil, fs.ErrNotExist
+}