feat: add Tavily as configurable web search provider

[tavily-integrations]

Summary

• Added Tavily as a new web search engine option in the online search module
• Tavily is used when TAVILY_API_KEY environment variable is set
• Existing search providers (Serper, Exa, Firecrawl, Google, SearXNG) are unchanged
• Tavily participates in the existing fallback chain automatically

Files changed

• src/khoj/processor/tools/online_search.py — Added TAVILY_API_KEY env var, search_with_tavily() async function using tavily-python SDK, and registered Tavily in the search engine dispatcher
• pyproject.toml — Added tavily-python >= 0.5.0 to project dependencies

Dependency changes

• Added tavily-python >= 0.5.0 to pyproject.toml

Environment variable changes

• Added TAVILY_API_KEY — set this to enable Tavily as a search provider

Notes for reviewers

• The search_with_tavily() function normalizes results to the same {organic: [{title, link, snippet, content}]} format used by all other providers
• Uses AsyncTavilyClient for async compatibility with the existing asyncio.gather pattern
• The import is done inside the function to avoid import errors when tavily-python is not installed or TAVILY_API_KEY is not set

🤖 Generated with Claude Code

Automated Review

• Passed after 1 attempt(s)
• Final review: The Tavily web search integration is well-implemented and additive. It correctly uses AsyncTavilyClient, maps result fields to the expected {"organic": [...]} format, registers in the dispatcher only when the API key is present, and adds the dependency to pyproject.toml. No regressions to existing providers. Three minor issues noted but none are blocking.