feat: add Tavily as configurable search engine alongside SearXNG

[tavily-integrations]

Summary

• Added Tavily as an opt-in search backend selectable via --engine tavily, keeping SearXNG as the default
• Created internal/search/tavily.go — HTTP client that POSTs to the Tavily /search REST API and maps results (title, url, content, score) into the existing SearchResult type
• Added TavilyAPIKey field to SearchConfig struct, loaded from TAVILY_API_KEY env var or tavily_api_key JSON config key
• Updated Search struct with engine-dispatch logic in Do() to route between SearXNG and Tavily
• Updated CLI --engine flag to accept tavily in addition to searxng

Files changed

• internal/config/config.go — Added TavilyAPIKey to SearchConfig
• internal/config/loader.go — Read TAVILY_API_KEY env var; propagate field in mergeSearchConfig
• internal/search/tavily.go — New file: Tavily REST API client
• internal/search/search.go — Added engine dispatch, TavilyClient field, refactored Do() into doTavily/doSearXNG
• cmd/web-search/main.go — Accept tavily as valid --engine value, pass engine to NewSearch

Dependency changes

• None (Tavily API called via stdlib net/http)

Environment variable changes

• Added TAVILY_API_KEY — required when using --engine tavily; SearXNG continues to work without it

Notes for reviewers

• SearXNG remains the default engine; no existing behaviour changes
• If --engine tavily is used without TAVILY_API_KEY set, a clear error is returned
• No new Go module dependencies — uses stdlib net/http for the Tavily REST API

🤖 Generated with Claude Code

Automated Review

• Passed after 2 attempt(s)
• Final review: All four issues from the previous review have been correctly addressed. The Tavily integration is now functionally complete: config is loaded via config.Load() (picking up env vars and JSON files), API auth uses Authorization: Bearer header, CLI descriptions document Tavily, and topic/time_range parameters are properly mapped. The config layer changes (internal/config/config.go and internal/config/loader.go) are correctly included in the diff despite not being listed in the implementation summary. SearXNG behaviour is fully preserved. No new dependencies are introduced — Tavily is implemented using the standard library only, which is consistent with the existing SearXNG client pattern. One minor issue noted below about Tavily topic validation.