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>

nanobot/webui/settings_api.py

@@ -90,6 +90,7 @@ _WEB_SEARCH_PROVIDER_OPTIONS: tuple[dict[str, str], ...] = (
     {"name": "olostep", "label": "Olostep", "credential": "api_key"},
     {"name": "bocha", "label": "Bocha", "credential": "api_key"},
     {"name": "volcengine", "label": "Volcengine Search", "credential": "api_key"},
+    {"name": "keenable", "label": "Keenable", "credential": "api_key"},
 )
 _WEB_SEARCH_PROVIDER_BY_NAME = {
     provider["name"]: provider for provider in _WEB_SEARCH_PROVIDER_OPTIONS