fix(ci): Fix backend-core TARGETARCH for amd64 + set -e in deploy

- backend-core Dockerfile defaults TARGETARCH=arm64, override with build arg
- Add set -e in helper container to fail fast on build errors

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitea/workflows/ci.yaml

@@ -212,8 +212,8 @@ jobs:
             -w "${DEPLOY_DIR}" \
             docker:27-cli \
             sh -c "
+              set -e
               COMPOSE_FILES='-f docker-compose.yml -f docker-compose.hetzner.yml'
-              SERVICES='postgres valkey qdrant minio ollama mailpit embedding-service rag-service backend-core consent-service health-aggregator'
 
               echo '=== Building Docker Images ==='
               docker compose \${COMPOSE_FILES} build --parallel \