feat: add OpenAI image generation controls

backend/internal/service/image_billing_multiplier.go

@@ -0,0 +1,11 @@
+package service
+
+func resolveImageRateMultiplier(apiKey *APIKey, effectiveGroupMultiplier float64) float64 {
+	if apiKey != nil && apiKey.Group != nil && apiKey.Group.ImageRateIndependent {
+		if apiKey.Group.ImageRateMultiplier < 0 {
+			return 0
+		}
+		return apiKey.Group.ImageRateMultiplier
+	}
+	return effectiveGroupMultiplier
+}