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/gemini/recipe.go

@@ -68,7 +68,7 @@ type NutritionInfo struct {
 func (c *Client) GenerateRecipes(ctx context.Context, req RecipeRequest) ([]Recipe, error) {
 	prompt := buildRecipePrompt(req)
 
-	// OpenAI-compatible messages format used by Groq.
+	// OpenAI messages format.
 	messages := []map[string]string{
 		{"role": "user", "content": prompt},
 	}