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/internal/config/config.go

@@ -22,7 +22,7 @@ type Config struct {
 	AllowedOrigins []string `envconfig:"ALLOWED_ORIGINS" default:"http://localhost:3000"`
 
 	// External APIs
-	GeminiAPIKey  string `envconfig:"GEMINI_API_KEY" required:"true"`
+	OpenAIAPIKey  string `envconfig:"OPENAI_API_KEY" required:"true"`
 	PexelsAPIKey  string `envconfig:"PEXELS_API_KEY" required:"true"`
 }