feat: add Tavily search provider alongside Google Custom Search

[tavily-integrations]

Summary

• Added Tavily Search REST API as a configurable alternative to Google Custom Search in SearchService
• New search.provider property (google or tavily, defaults to google) controls which provider is used
• When Tavily is active, raw_content from the API response is used directly, bypassing Jsoup scraping
• Added automatic Tavily fallback: if Google returns no results and tavily.api.key is configured, Tavily is tried automatically
• Google Custom Search path remains completely unchanged

Files changed

• src/main/java/org/example/aisearchapp/service/SearchService.java — Added tavilyApiKey and searchProvider fields, searchTavily() method, and provider branching in getAiSummary()
• src/main/resources/application.properties.example — Added search.provider and tavily.api.key properties

Dependency changes

• No new JAR dependencies — Tavily REST API is consumed via the existing RestTemplate and ObjectMapper

Environment variable changes

• TAVILY_API_KEY → maps to tavily.api.key Spring property
• SEARCH_PROVIDER → maps to search.provider (values: google or tavily, default google)

Notes for reviewers

• The Tavily API is called with include_raw_content=true and max_results=5 to match the existing Google path's behavior
• Blocked source filtering (isBlockedSource) is applied to Tavily results as well
• Content is truncated to 4000 chars per result, consistent with the existing scraping logic

🤖 Generated with Claude Code

Automated Review

• Passed after 1 attempt(s)
• Final review: The migration is functionally correct and well-scoped. Tavily is called via a proper POST to https://api.tavily.com/search with valid parameters (api_key, query, include_raw_content, max_results), and the response is parsed correctly using results[].url and results[].raw_content. The additive strategy is cleanly implemented — Google+Jsoup is the preserved default, Tavily is an opt-in provider via search.provider=tavily, and a Google-to-Tavily fallback is added when Google returns empty results. application.properties.example is updated with the two new properties and a helpful comment. No regressions, no dead code, no broken imports. Three minor issues found but none block approval.