Add .zip extension to coverage report artifact name

This makes the artifact more recognizable in the Forgejo UI as a downloadable zip file.

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

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

.forgejo/workflows/ci.yml

@@ -94,7 +94,7 @@ jobs:
       - name: Upload coverage reports
         uses: https://code.forgejo.org/forgejo/upload-artifact@v4
         with:
-          name: coverage-report
+          name: coverage-report.zip
           path: coverage/
           
   build: