feat: add Tavily search provider option to Express API

[manisrinivasan2k1]

Summary

Adds Tavily as a configurable, parallel search provider in the Express API while keeping the existing BraveSearch integration as the default.

• Imported @tavily/core and added a Tavily search path in searchEngineForSources()
• Provider is selected via SEARCH_PROVIDER env var ('brave' default, 'tavily' to use Tavily)
• Tavily results are normalized to { title, link } to match the existing BraveSearch output format

Files changed

• express-api/index.js — Added Tavily import and conditional search path gated by SEARCH_PROVIDER
• express-api/package.json — Added @tavily/core dependency
• express-api/.env.example — Added TAVILY_API_KEY and SEARCH_PROVIDER entries

Dependency changes

• Added @tavily/core ^0.0.7 to express-api/package.json

Environment variable changes

• Added TAVILY_API_KEY — required when SEARCH_PROVIDER=tavily
• Added SEARCH_PROVIDER — optional, defaults to brave

Notes for reviewers

• BraveSearch remains the default; no breaking changes
• The Tavily path reuses the existing fetchAndProcess pipeline (cheerio scraping + vectorization) for consistency

Automated Review

• Passed after 1 attempt(s)
• Final review: The migration correctly adds Tavily as a configurable search provider alongside BraveSearch in the Express API. All three files listed in the plan are modified. The Tavily SDK is imported and used with correct patterns, the dependency is added to package.json, and environment variables are documented in .env.example. No regressions to the existing Brave path are introduced. A few minor issues exist but none are blocking.

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