fix(helm): switch migrate init container from busybox to alpine

busybox:1.37 nc -z is broken (outputs "punt!" and hangs).
Alpine 3.21 ships a working nc -z implementation.

backend/deploy/helm/templates/migrate-job.yaml

@@ -34,7 +34,7 @@ spec:
       initContainers:
         # Wait for CNPG to provision the cluster and secret before migrating
         - name: wait-for-postgres
-          image: busybox:1.37
+          image: alpine:3.21
           resources:
             requests:
               cpu: 10m
@@ -52,7 +52,7 @@ spec:
             - -c
             - |
               echo "Waiting for PostgreSQL..."
-              until nc -w 2 "$DB_HOST" "$DB_PORT" < /dev/null; do
+              until nc -z -w 2 "$DB_HOST" "$DB_PORT" 2>/dev/null; do
                 sleep 3
               done
               echo "PostgreSQL is reachable"