ci: build backend with system chromium for the arm64 crawler

chromiumoxide's fetcher has no linux/arm64 build, so on the Pi the crawler's
browser launch failed with 'OS linux aarch64 is not supported'. Build the
backend image with INSTALL_CHROMIUM=true (Debian chromium-headless-shell,
verified present + runnable on trixie/arm64 at /usr/bin/chromium-headless-shell)
and set CRAWLER_CHROMIUM_BINARY to it in .env so the launcher uses the system
binary instead of the fetcher. Backend image only; frontend unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/deploy.yml

@@ -113,7 +113,13 @@ jobs:
           echo "$REGISTRY_PASSWORD" | docker login "$REGISTRY_URL" -u "$REGISTRY_USERNAME" --password-stdin
           for svc in backend frontend; do
             img="$REGISTRY_URL/mangalord-$svc"
-            docker build -t "$img:$IMAGE_TAG" -t "$img:latest" -t "$img:$VERSION" "./$svc"
+            # The backend bundles Debian's headless chromium for the crawler:
+            # chromiumoxide's fetcher has no arm64 build, so the Pi must use a
+            # system binary (pair with the runtime env
+            # CRAWLER_CHROMIUM_BINARY=/usr/bin/chromium-headless-shell).
+            build_args=""
+            if [ "$svc" = "backend" ]; then build_args="--build-arg INSTALL_CHROMIUM=true"; fi
+            docker build $build_args -t "$img:$IMAGE_TAG" -t "$img:latest" -t "$img:$VERSION" "./$svc"
             for tag in "$IMAGE_TAG" latest "$VERSION"; do docker push "$img:$tag"; done
           done
           docker logout "$REGISTRY_URL"