ci: downgrade upload-artifact to v3 for Forgejo compatibility

actions/upload-artifact@v4 requires GHES features not available in
self-hosted Forgejo, causing artifact upload failures with:
GHESNotSupportedError: upload-artifact@v4+ are not supported on GHES.

- Downgrade both coverage upload steps from v4 to v3
- v3 uses a compatible upload mechanism that works on Forgejo

Keeps the artifact upload functionality so coverage HTML reports remain
downloadable from the workflow run page.

.forgejo/workflows/ci.yml

@@ -47,14 +47,14 @@ jobs:
         working-directory: frontend
 
       - name: Upload backend coverage report
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v3
         with:
           name: backend-coverage
           path: backend/build/reports/jacoco/test/html/
           retention-days: 7
 
       - name: Upload frontend coverage report
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v3
         with:
           name: frontend-coverage
           path: frontend/coverage/