feat: add Tavily search provider to config and API layer

[tavily-integrations]

Summary

Adds Tavily as a configurable search provider alongside existing providers (SearXNG, Brave, SerpAPI, DuckDuckGo). This is an additive change — no existing provider logic is removed or modified.

What changed

• server/src/services/ConfigManager.ts: Extended AppConfig.search.preferred_provider union type to include 'tavily'; added tavily_api_key: string field with empty-string default.
• server/src/routes/searchRoutes.ts: Accept tavily_api_key in POST /api/search/providers/config body; call configureProvider('tavily', ...) when provided; added 'tavily' to the priority maps for both preferred-provider promotion and auto-mode default priorities.
• server/src/index.ts: On startup, read TAVILY_API_KEY from env (or saved config) and call configureProvider('tavily', { apiKey, enabled: true }); added 'tavily' to the preferred-provider priority bump map.
• .env.example: Added TAVILY_API_KEY= entry under the Web Search section.

Files changed

• server/src/services/ConfigManager.ts
• server/src/routes/searchRoutes.ts
• server/src/index.ts
• .env.example

Dependency changes

None — Tavily provider implementation (SDK dependency) is handled in a separate migration unit.

Environment variable changes

• Added TAVILY_API_KEY reference in .env.example and startup config loading

Notes for reviewers

• All pre-existing TypeScript errors are unrelated to these changes (verified via tsc --noEmit).
• The actual Tavily search provider implementation (SDK integration) is expected in a separate migration unit — this unit only wires the config/API plumbing.

Automated Review

• Passed after 2 attempt(s)
• Final review: The config-api-tavily-support unit is well-implemented. All four changed files (ConfigManager.ts, searchRoutes.ts, index.ts, .env.example) are consistent and correct. The Tavily priority of 3 in the auto-reset block matches the canonical order from the prerequisite unit (searxng=1, tavily=3, serpapi=5, brave=10, duckduckgo=100). The as string cast in index.ts is valid since the enclosing condition guarantees a truthy value. No regressions, no dead code, and no unintended changes were found.