Fix CI artifact upload compatibility with GHES

- Downgrade upload-artifact action from v4 to v3 for GHES compatibility
- Resolves artifact upload error in coverage reports step

🤖 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@v4
+        uses: actions/upload-artifact@v3
         with:
           name: coverage-report
           path: coverage/