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/menu/handler.go

@@ -135,7 +135,7 @@ func (h *Handler) GenerateMenu(w http.ResponseWriter, r *http.Request) {
 		menuReq.AvailableProducts = products
 	}
 
-	// Generate 7-day plan via Groq.
+	// Generate 7-day plan via OpenAI.
 	days, err := h.gemini.GenerateMenu(r.Context(), menuReq)
 	if err != nil {
 		slog.Error("generate menu", "user_id", userID, "err", err)