fix(cluster): 兼容 CNPG 单实例预检

发布前先通过 merge patch 删除旧同步复制配置，期望清单不再写入 CRD 不接受的 null 值，确保 server-side dry-run 与正式 apply 使用相同的有效单实例状态。

deploy/kubernetes/easyai-ai-gateway-cluster-release

@@ -716,7 +716,7 @@ prepare_postgres_single_instance() {
   local source_sha=$1
   local current_instances current_primary primary_node synchronous_enabled
   if ! grep -Eq '^  instances: 1$' "$DESIRED_STATE_DIR/database.yaml" ||
-     ! grep -Eq '^    synchronous: null$' "$DESIRED_STATE_DIR/database.yaml"; then
+     grep -Eq '^[[:space:]]+synchronous:' "$DESIRED_STATE_DIR/database.yaml"; then
     echo 'production PostgreSQL desired state must declare one instance with synchronous replication disabled' >&2
     return 1
   fi

deploy/kubernetes/production/database.yaml

@@ -117,9 +117,6 @@ spec:
       cpu: "2"
       memory: 3Gi
   postgresql:
-    # Keep this explicit null so server-side apply atomically removes the old
-    # two-instance synchronous setting during preflight field management.
-    synchronous: null
     parameters:
       # A 16 MiB WAL segment every 60 seconds requires at least 2.24 Mbit/s
       # of continuous cross-region throughput even while the database is idle.