websearch: SearXNG 默认只查 bing,360search

默认 general 类别在 Docker 任务容器里要 20.0 秒才回
（wikipedia/wikidata/brave/duckduckgo/google 引擎全超时，
只有 bing+360search 回），撞 MCP 默认 20 秒超时；随后
DuckDuckGo 经代理也超时，整条链失败。代理本身无问题
（Tinyproxy 白名单早已放行 searx 与 duckduckgo）。
修法：_provider_searxng 默认 engines=bing,360search，
SEARXNG_ENGINES 可覆盖、空值恢复旧行为；worker 搜索
env 透传补 SEARXNG_ENGINES。容器实测：钉住后 0.26 秒
回 3 条（首条北京天气），auto 链 0.15 秒成功；旧 20.0 秒。
测试：新增 test_websearch_searxng 3 项，MCP/worker 共
21 项全过。生产未部署。

cloud/websearch.py

@@ -267,8 +267,22 @@ def _provider_searxng(query, limit, env, http):
     base = (env.get("SEARXNG_URL") or "").rstrip("/")
     if not base:
         raise WebSearchError("searxng not configured (SEARXNG_URL unset)")
-    url = (base + "/search?"
-           + urllib.parse.urlencode({"q": query, "format": "json"}))
+    # Engines are pinned, not left to SearXNG's default "general"
+    # category. Measured 2026-10-30 on the production host:
+    # default general search took 20.0s in a Docker task container
+    # (wikipedia / wikidata / brave / duckduckgo / google engines
+    # all timing out, only bing + 360search returning), which hits
+    # the MCP's 20s timeout and then falls through to DuckDuckGo,
+    # which itself times out via the egress proxy -- so the whole
+    # chain fails even though SearXNG can answer. Pinning to the
+    # two proven-fast engines cuts the same query to 0.47s via
+    # the same proxy. SEARXNG_ENGINES overrides the pin; an empty
+    # value restores the old unpinned behaviour.
+    params = {"q": query, "format": "json"}
+    engines = env.get("SEARXNG_ENGINES", "bing,360search")
+    if engines:
+        params["engines"] = engines
+    url = base + "/search?" + urllib.parse.urlencode(params)
     _status, body, _ctype = http.get(url, _default_headers(env, "searxng"))
     data = json.loads(body.decode("utf-8", errors="replace"))
     results = []