Fix CI: use upload-artifact@v3, hardcode public Gitea hostname

upload-artifact@v4 is not supported on self-hosted Gitea (GHES).
Downgrade to v3 which works with Gitea Actions.

GITHUB_SERVER_URL resolves to the internal Docker address (http://gitea:3000)
instead of the public URL, making the gh-pages push URL malformed.
Hardcode gitea.archvium.eu as the push target.

.gitea/workflows/ci.yml

@@ -38,7 +38,7 @@ jobs:
           echo '```' >> "$GITHUB_STEP_SUMMARY"
 
       - name: Upload firmware artifacts
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v3
         with:
           name: firmware-${{ github.sha }}
           path: |
@@ -110,7 +110,6 @@ jobs:
           git config user.email "ci@localhost"
           git add -A
           git commit -m "docs: ${GITHUB_SHA}"
-          SERVER="${GITHUB_SERVER_URL#https://}"
           git push --force \
-            "https://x-token:${GITHUB_TOKEN}@${SERVER}/${GITHUB_REPOSITORY}.git" \
+            "https://x-token:${GITHUB_TOKEN}@gitea.archvium.eu/${GITHUB_REPOSITORY}.git" \
             HEAD:gh-pages