Use Forgejo-specific upload-artifact action

- Replace GitHub Actions upload-artifact with Forgejo's patched version
- Use https://code.forgejo.org/forgejo/upload-artifact@v4 for compatibility
- Should resolve artifact upload issues on Forgejo server

🤖 Generated with [Claude Code](https://claude.ai/code)

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

.forgejo/workflows/ci.yml

@@ -92,7 +92,7 @@ jobs:
         run: npm run test:coverage
         
       - name: Upload coverage reports
-        uses: actions/upload-artifact@v3
+        uses: https://code.forgejo.org/forgejo/upload-artifact@v4
         with:
           name: coverage-report
           path: coverage/