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>

.woodpecker/build.yaml

@@ -6,7 +6,7 @@ steps:
   - name: test
     image: golang:1.24-alpine
     commands:
-      - go test ./...
+      - go test -tags testing ./...
 
   - name: build
     image: docker:27-cli