Fix xAI sentiment: use Agent Tools web_search (Live Search deprecated)

xAI returned 410 — search_parameters/Live Search is retired. Route xAI
through the Responses API web_search tool instead (same path as OpenAI):
- OpenAISentimentProvider parametrized with base_url / tool_type / source
- xAI builds it against https://api.x.ai/v1 with the web_search tool
- Drop the dead Live Search code from the generic compatible provider
- Frontend label: "xAI Grok — web search"

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/components/admin/SentimentProviderSettings.tsx

@@ -18,7 +18,7 @@ const PROVIDER_LABELS: Record<string, string> = {
   openai: 'OpenAI — web search',
   gemini: 'Google Gemini — web search',
   deepseek: 'DeepSeek — cheap, no web search',
-  xai: 'xAI Grok — Live Search',
+  xai: 'xAI Grok — web search',
   openai_compatible: 'OpenAI-compatible — custom URL',
 };