arrstack: gate apps on DB via initContainer instead of sync-waves

Per review: drop the ArgoCD sync-wave annotations (their health-gating is
what deadlocked the first deploy) and instead add a wait-for-db init
container to each app that blocks until its own Postgres database+role is
reachable (libpq PG* env, password never in argv). ArgoCD applies
everything at once; the app container only starts once its DB is ready.

apps/base/arrstack/postgres/cnpg_backup.yaml

@@ -7,9 +7,6 @@ kind: ObjectStoreUser
 metadata:
   name: cnpg-arrstack-backup
   namespace: arrstack
-  annotations:
-    # S3 creds Secret must exist before the Cluster (wave -2) reconciles its backup.
-    argocd.argoproj.io/sync-wave: "-3"
 spec:
   displayName: "CNPG backup owner (arrstack)"
   uid: cnpg-arrstack-backup
@@ -22,8 +19,6 @@ kind: Bucket
 metadata:
   name: cnpg-arrstack
   namespace: arrstack
-  annotations:
-    argocd.argoproj.io/sync-wave: "-3"
 spec:
   placementTarget: ec
   bucketName: cnpg-arrstack