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>

docker-compose.hetzner.yml

@@ -50,6 +50,11 @@ services:
 
   backend-core:
     platform: linux/amd64
+    build:
+      context: ./backend-core
+      dockerfile: Dockerfile
+      args:
+        TARGETARCH: amd64
     ports:
       - "8000:8000"
     environment: