feat(gateway): 接口5排队插队优先级，两级队列均支持

- 全局槽换为 _PrioritySemaphore：priority 大者排到等待队列最前，同级 FIFO，
  无等待者时才允许直接获取（收紧 asyncio.Semaphore 的 barging）；
  授予槽位瞬间请求被取消时归还槽位，防泄漏
- worker 池 Condition FIFO 等待改为显式优先级等待队列，release 唤醒队首；
  被唤醒但 worker 被抢时按原序号重排；授予/取消竞态时转手唤醒下一位
- forward.py 新增 PRIORITY_PATHS（现仅接口5 /api/v1/hairline/generate）
- 新增 4 个单测：两级队列的插队 + 同级 FIFO

gateway/forward.py

@@ -29,6 +29,15 @@ from gateway.reqlog import extract_form_params
 
 logger = logging.getLogger("gateway.forward")
 
+# 插队优先级接口：排队（全局槽 / worker 池）时排到等待队列最前，同优先级 FIFO。
+PRIORITY_PATHS = frozenset({
+    "/api/v1/hairline/generate",  # 接口5：发际线PNG生成
+})
+
+
+def _priority_of(path: str) -> int:
+    return 1 if path in PRIORITY_PATHS else 0
+
 
 # ---------------------------------------------------------------------------
 # 全局 httpx client（连接复用）
@@ -137,7 +146,8 @@ async def proxy_request(request: Request, path: str) -> JSONResponse:
     """代理一次请求到 worker，处理重试与 base64 改写。
 
     流程：
-    1. acquire worker（排队等待空闲）
+    0. 获取全局串行槽（PRIORITY_PATHS 中的接口插队到最前）
+    1. acquire worker（排队等待空闲，同样支持优先级插队）
     2. 重构 multipart/form 请求，加 X-Internal-Token
     3. 转发到 worker
     4. 成功 → release worker → 改写 base64 → 返回 JSONResponse
@@ -161,8 +171,9 @@ async def proxy_request(request: Request, path: str) -> JSONResponse:
     except Exception as ex:  # noqa: BLE001
         request.state.log_request_params = {"_parse_error": str(ex)}
 
-    # --- 全局串行槽（GPU 请求在此排队；超时拒绝） ---
-    if not await acquire_global_slot(cfg):
+    # --- 全局串行槽（GPU 请求在此排队；超时拒绝；优先级接口插队到最前） ---
+    priority = _priority_of(path)
+    if not await acquire_global_slot(cfg, priority):
         request.state.worker_url = ""
         return JSONResponse(
             status_code=503,
@@ -175,13 +186,13 @@ async def proxy_request(request: Request, path: str) -> JSONResponse:
         )
 
     try:
-        return await _dispatch_with_retries(request, path, cfg, body)
+        return await _dispatch_with_retries(request, path, cfg, body, priority)
     finally:
         release_global_slot()
 
 
 async def _dispatch_with_retries(
-    request: Request, path: str, cfg: dict, body: bytes
+    request: Request, path: str, cfg: dict, body: bytes, priority: int = 0
 ) -> JSONResponse:
     """在已持有全局串行槽的前提下：选 worker → 转发 → 重试 → 改写 base64 → 返回。
 
@@ -203,7 +214,7 @@ async def _dispatch_with_retries(
     for attempt in range(attempts):
         worker = None
         try:
-            worker = await acquire_worker(cfg)
+            worker = await acquire_worker(cfg, priority)
             # 记录当前使用的 worker，供日志中间件读取
             request.state.worker_url = worker.url
         except NoWorkerAvailable: