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/cmd/server/main.go

@@ -93,7 +93,7 @@ func run() error {
 	authMW := middleware.Auth(&jwtAdapter{jm: jwtManager})
 
 	// External API clients
-	geminiClient := gemini.NewClient(cfg.GeminiAPIKey)
+	geminiClient := gemini.NewClient(cfg.OpenAIAPIKey)
 	pexelsClient := pexels.NewClient(cfg.PexelsAPIKey)
 
 	// Ingredient domain