feat: migrate search to support Tavily as configurable provider

[tavily-integrations]

Summary

• Added Tavily as an opt-in search provider alongside the existing DuckDuckGo provider
• New search() dispatch function routes to Tavily when SEARCH_PROVIDER=tavily and TAVILY_API_KEY is set, otherwise falls back to DuckDuckGo
• Updated /v1/search route, MCP SSE server, and standalone MCP server to use the provider-aware dispatch
• DuckDuckGo remains the default — no breaking changes

Files changed

• src/services/search.ts — Added searchTavily() and search() dispatch function
• src/routes/search.ts — Switched from searchDuckDuckGo to search dispatch
• src/mcp-sse.ts — Switched to search dispatch, updated tool description
• src/mcp-server.ts — Switched to search dispatch, updated tool description
• package.json — Added @tavily/core dependency
• .env.example — Documented SEARCH_PROVIDER and TAVILY_API_KEY

Dependency changes

• Added @tavily/core ^0.6.4 (npm)

Environment variable changes

• SEARCH_PROVIDER (optional, default: duckduckgo, values: duckduckgo | tavily)
• TAVILY_API_KEY (required when SEARCH_PROVIDER=tavily)

Notes for reviewers

• The Tavily SDK uses content field where DuckDuckGo uses snippet — the mapping is handled in searchTavily()
• TypeScript compiles with zero errors
• No changes to the response contract — SearchResult interface is unchanged

🤖 Generated with Claude Code

Automated Review

• Passed after 1 attempt(s)
• Final review: The Tavily migration is correct and complete. All 7 files listed in the plan are updated: the dispatch function in src/services/search.ts correctly routes to Tavily when configured and falls back to DuckDuckGo otherwise, all callers (route handler, mcp-sse, mcp-server) are updated to use the new search export, @tavily/core is properly added as a dependency, and env vars are documented in .env.example. The Tavily SDK usage (tavily() client + client.search()) is correct. Three minor issues exist but none block approval.