feat(web): register Keenable provider in WebUI, docs, and tests

Bring the Keenable search provider in line with the established
multi-file provider pattern so it surfaces everywhere the others do:

- settings_api.py: register in the web-search provider options so it
  appears in the WebUI settings dropdown (credential: api_key, optional).
- WebUI provider-brand: add keenable brand entry + brand test.
- docs/configuration.md: provider table row + config example.
- Harden _search_keenable: honor config.timeout and return explicit
  messages on HTTP status errors (429 / other), matching peer providers.
- Add env-key and HTTP-error tests; add the websocket settings whitelist
  assertion.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/tools/test_web_search_tool.py

@@ -178,6 +178,32 @@ async def test_keenable_search_without_key_omits_header(monkeypatch):
     assert "Anon" in result
 
 
+@pytest.mark.asyncio
+async def test_keenable_search_uses_env_api_key(monkeypatch):
+    async def mock_post(self, url, **kw):
+        assert kw["headers"]["X-API-Key"] == "env-keen-key"
+        return _response(json={
+            "results": [{"title": "Env", "url": "https://keenable.ai/env", "description": "ok"}]
+        })
+
+    monkeypatch.setattr(httpx.AsyncClient, "post", mock_post)
+    monkeypatch.setenv("KEENABLE_API_KEY", "env-keen-key")
+    tool = _tool(provider="keenable", api_key="")
+    result = await tool.execute(query="keenable", count=1)
+    assert "Env" in result
+
+
+@pytest.mark.asyncio
+async def test_keenable_search_http_error(monkeypatch):
+    async def mock_post(self, url, **kw):
+        return _response(status=401, json={"error": "invalid key"})
+
+    monkeypatch.setattr(httpx.AsyncClient, "post", mock_post)
+    tool = _tool(provider="keenable", api_key="bad-keen-key")
+    result = await tool.execute(query="keenable")
+    assert "Error: Keenable search failed (401)" in result
+
+
 @pytest.mark.asyncio
 async def test_bocha_search(monkeypatch):
     async def mock_post(self, url, **kw):