fix: CI artifact upload/download v4→v3 (Gitea ne supporte pas v4)

actions/upload-artifact@v4 et download-artifact@v4 utilisent une API
immatriculée GitHub Actions qui n'existe pas sur Gitea.
v3 est compatible et supporté.
Source: https://gitea.com/actions/gitea-upload-artifact

.gitea/workflows/ci.yaml

@@ -103,7 +103,7 @@ jobs:
 
       - name: Upload web artifact
         if: github.ref == 'refs/heads/main' && github.event_name == 'push'
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v3
         continue-on-error: true
         with:
           name: web-artifact
@@ -125,7 +125,7 @@ jobs:
           git reset --hard origin/main
 
       - name: Download web artifact
-        uses: actions/download-artifact@v4
+        uses: actions/download-artifact@v3
         continue-on-error: true
         with:
           name: web-artifact