feat: add Tavily as alternative search provider alongside Perplexity

[tavily-integrations]

Summary

Adds Tavily as a configurable search backend alongside the existing Perplexity AI provider. The search provider is selected via the SEARCH_PROVIDER environment variable (perplexity by default, or tavily).

What changed

• src/perplexity_mcp/server.py: Added call_tavily() function using the tavily-python SDK (TavilyClient.search() with search_depth="advanced" and time_range mapped from the existing recency parameter). Updated call_tool dispatcher to branch on SEARCH_PROVIDER env var. Updated main() and main_async() startup validation to require TAVILY_API_KEY when SEARCH_PROVIDER=tavily, and log which provider is active.
• pyproject.toml: Added tavily-python>=0.5 to project dependencies.
• README.md: Documented SEARCH_PROVIDER and TAVILY_API_KEY environment variables and updated config example.

Dependency changes

• Added tavily-python>=0.5 to pyproject.toml dependencies

Environment variable changes

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

Notes for reviewers

• This is an additive change — all existing Perplexity code paths and env vars are preserved unchanged.
• The Tavily time_range parameter accepts the same values (day, week, month, year) as the existing recency parameter, so no mapping conversion is needed.
• Tavily results are formatted as a numbered list with title, URL, and content snippet per result.

Automated Review

• Passed after 2 attempt(s)
• Final review: The migration correctly adds Tavily as an additive, opt-in search provider behind a SEARCH_PROVIDER env var. AsyncTavilyClient is used and properly awaited, the recency→time_range mapping is 1:1 with Tavily's accepted enum values (day/week/month/year), error handling is present, pyproject.toml gains the tavily-python>=0.5 dependency, and README documents the new env vars. Existing Perplexity behavior is fully preserved as the default path. No critical or major issues found.