Fix model, improve error handling, add startup diagnostics for all modes

Complete fix: All modes now use Tavily fallback, always show images/sources/links

Major fix: Improved web search with Tavily AI answers, better content extraction, enhanced system prompt

Fix: LangGraph node name conflicts and port binding

Add Product MVP and Video Brain modes + Render deployment

Perplexity AI Clone - Full Production Version with 8 Modes