feat: add Tavily as configurable search provider alongside SearxNG

[tavily-integrations]

Summary

• Added Tavily as an optional search provider that can be used alongside the existing SearxNG integration
• Search provider is selected at runtime via the SEARCH_PROVIDER env var (defaults to searxng)
• Tavily integration uses direct reqwest HTTP calls to https://api.tavily.com/search — no new Cargo dependencies needed
• Tavily's response naturally returns results[].url, matching the shape process_search_results already consumes

Files changed

• src/api.rs — Added tavily_search() async function; modified search_and_index() to branch on SEARCH_PROVIDER env var
• src/search.rs — Added TavilyResult and TavilyResponse structs for typed deserialization
• sample.env — Added SEARCH_PROVIDER=searxng (default) and TAVILY_API_KEY=<your_key>

Dependency changes

• None — reqwest with JSON feature is already in Cargo.toml

Environment variable changes

• Added SEARCH_PROVIDER (default: searxng; set to tavily to use Tavily)
• Added TAVILY_API_KEY (required when SEARCH_PROVIDER=tavily)

Notes for reviewers

• SearxNG code path is completely preserved and remains the default
• Rust toolchain was not available in the CI environment for compilation verification; please confirm cargo check passes
• Tavily API key is passed in the POST body per the Tavily REST API convention

Automated Review

• Passed after 2 attempt(s)
• Final review: All 4 review fixes from the re-implementation plan are correctly addressed. Bearer token auth is valid for Tavily API. The raw Tavily response shape ({"results": [{"url": "..."}]}) is already compatible with process_search_results — no normalization needed. No new Cargo dependencies required. SearxNG remains the default (backward compatible). Only minor doc comment issues remain, nothing blocking.