feat(gateway): worker 池主备分组派发，主全挂才用备机

- pool.py: WorkerState 增加 primary 标记；acquire_worker 分级派发——
  主组有在线只在主组内挑空闲（全忙排队，不退备组），主组全部下线才用备机
- config.py: 新增可选 primary_workers 配置（须为 workers 子集，自动过滤+告警）；
  不配置/空列表 = 全部视为主 worker，兼容旧行为
- config.json(不入 git): 3 台 GPU worker 设为主，max_global_concurrency 1→3
- app.py: /health 增加 workers_primary_total / workers_primary_healthy
- tests: 新增 4 条主备派发单测（主优先/主全挂用备/主全忙不走备/无配置兼容）
- docs(实现说明): 补充 primary_workers 配置说明，改配置需重启网关生效

gateway/app.py

@@ -123,6 +123,8 @@ async def gateway_health():
         "workers_total": status["total"],
         "workers_healthy": status["healthy"],
         "workers_busy": status["busy"],
+        "workers_primary_total": status.get("primary_total", 0),
+        "workers_primary_healthy": status.get("primary_healthy", 0),
         "global_max": status.get("global_max", 1),
         "global_busy": status.get("global_busy", 0),
         "global_waiting": status.get("global_waiting", 0),