Fix CI: downgrade artifact actions to v3 for Gitea Actions compatibility

upload-artifact@v4/download-artifact@v4 use the newer Artifact API v2,
which Gitea's act_runner doesn't support yet (GHESNotSupportedError).
v3 uses the older, Gitea-compatible API.

.gitea/workflows/pipeline.yml

@@ -27,7 +27,7 @@ jobs:
         # zijn zonder een draaiende Streamlit-sessie.
         run: pytest tests/ -v --cov=. --cov-report=term-missing --cov-report=xml
       - name: Bewaar coverage-rapport
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v3
         with:
           name: coverage-report
           path: coverage.xml
@@ -43,7 +43,7 @@ jobs:
         with:
           fetch-depth: 0 # SonarQube wil de volledige git-historie zien
       - name: Download coverage-rapport
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v3
         with:
           name: coverage-report
       - name: SonarQube scan