feat: switch AI provider from Groq to OpenAI

Replace Groq/Llama with OpenAI API:
- Text model: gpt-4o-mini
- Vision model: gpt-4o
- Rename GEMINI_API_KEY → OPENAI_API_KEY env var
- Rename callGroq → callOpenAI, update all related constants and comments

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/.env.example

@@ -14,5 +14,5 @@ PORT=8080
 ALLOWED_ORIGINS=http://localhost:3000
 
 # External APIs
-GEMINI_API_KEY=your-gemini-key
+OPENAI_API_KEY=your-openai-key
 PEXELS_API_KEY=your-pexels-key