feat: add Tavily search provider option to settings and setup wizard

[tavily-integrations]

Summary

Adds Tavily as a configurable search provider alongside existing providers (SearXNG, Brave, DuckDuckGo) in the frontend settings UI. This is an additive change — no existing providers are removed or modified.

What changed

SettingsView.tsx:

• Added tavilyApiKey state variable
• Added tavily option to the preferred search provider dropdown
• Added Tavily API Key input field with link to https://app.tavily.com for key creation
• Wired tavily_api_key through the existing POST /api/search/providers/config call

SetupWizard.tsx:

• Added Tavily Search as an optional integration card in the setup wizard's integrations step (Step 3), parallel to the existing Brave Search entry

Files changed

• client/src/components/SettingsView.tsx
• client/src/components/SetupWizard.tsx

Dependency changes

None — purely frontend UI additions using existing components.

Environment variable changes

• TAVILY_API_KEY is now accepted via the UI and sent to the backend config endpoint as tavily_api_key

Notes for reviewers

• All changes are additive; existing Brave/SearXNG/DuckDuckGo fields remain untouched
• Backend must handle the new tavily_api_key field in the config endpoint (handled by a separate migration unit)
• Tavily card in SetupWizard is clearly labeled as optional

Automated Review

• Passed after 2 attempt(s)
• Final review: The unit correctly implements the frontend UI changes for Tavily: adds a tavilyApiKey state and password input in SettingsView, includes tavily_api_key in the POST body to /api/search/providers/config, adds "Tavily" to the preferred provider dropdown, adds Tavily to the SetupWizard INTEGRATIONS list, and maps tavily → TAVILY_API_KEY in the setupRoutes.ts integrationKeyMap. All three changed files are within scope and the changes are internally consistent. The backend handling of tavily_api_key in searchRoutes.ts (extraction, ConfigManager persistence, provider activation, priority management) is expected to land via the prerequisite config-api-tavily-support unit and should not block this unit.