feat(delivery): support multi-instance scheduling on a single host

- Per-host instance limit (DELIVERY_HOST_INSTANCE_LIMIT, default 4)
- Optional target_host to pin a host from the candidate pool
- Regenerate swagger docs

server/.env.example

@@ -22,6 +22,8 @@ DELIVERY_RESERVATION_TTL_MINUTES=120
 DELIVERY_GLOBAL_LIMIT=2
 DELIVERY_TARGET_LIMIT=2
 DELIVERY_BUSINESS_LIMIT=1
+# 单机 MySQL 实例数上限（同机多实例，容量由配额 + playbook 实机守卫兜底）
+DELIVERY_HOST_INSTANCE_LIMIT=4
 # 数据盘挂载点白名单（逗号分隔，第一项为默认值）
 DELIVERY_DATA_DISKS=/data
 AWX_BASE_URL=