Fix GitLab CI backend health check

Use docker exec curl instead of node http.get from host.
Port 3001 is not exposed to host, so health checks must run inside the container.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitlab-ci.yml

@@ -135,7 +135,7 @@ verify:
     - |
       HEALTH_OK=0
       for i in 1 2 3 4 5 6; do
-        if node -e "require('http').get('http://localhost:3001/health', (res) => { process.exit(res.statusCode === 200 ? 0 : 1) }).on('error', () => process.exit(1))" > /dev/null 2>&1; then
+        if docker exec mvp-backend curl -sf http://localhost:3001/health > /dev/null 2>&1; then
           echo "OK: Backend health check passed"
           HEALTH_OK=1
           break