feat: implement invisible 12% buyer fee

Business model:
- Seller registers R$10,00 → Seller sees R$10,00 → Seller receives R$10,00
- Buyer searches → sees R$11,20 (+12%) → pays R$11,20
- Marketplace keeps R$1,20 (12%)

Changes:
- config.go: Add BuyerFeeRate (default 0.12)
- handler.go: Add buyerFeeRate field to Handler struct
- product_handler.go: SearchProducts inflates prices for buyers
- server.go: Pass cfg.BuyerFeeRate to handler.New()
- handler_test.go: Fix 3 New() calls with fee rate

Env var: BUYER_FEE_RATE (default: 0.12)

backend/internal/config/config.go

@@ -17,6 +17,7 @@ type Config struct {
 	DatabaseURL           string
 	MercadoPagoBaseURL    string
 	MarketplaceCommission float64
+	BuyerFeeRate          float64 // Fee rate applied to buyer prices (e.g., 0.12 = 12%)
 	JWTSecret             string
 	JWTExpiresIn          time.Duration
 	PasswordPepper        string
@@ -40,6 +41,7 @@ func Load() Config {
 		DatabaseURL:           getEnv("DATABASE_URL", "postgres://postgres:postgres@localhost:5432/saveinmed?sslmode=disable"),
 		MercadoPagoBaseURL:    getEnv("MERCADOPAGO_BASE_URL", "https://api.mercadopago.com"),
 		MarketplaceCommission: getEnvFloat("MARKETPLACE_COMMISSION", 2.5),
+		BuyerFeeRate:          getEnvFloat("BUYER_FEE_RATE", 0.12), // 12% invisible fee
 		JWTSecret:             getEnv("JWT_SECRET", "dev-secret"),
 		JWTExpiresIn:          getEnvDuration("JWT_EXPIRES_IN", 24*time.Hour),
 		PasswordPepper:        getEnv("PASSWORD_PEPPER", ""),

backend/internal/http/handler/handler.go

@@ -16,11 +16,12 @@ import (
 var json = jsoniter.ConfigCompatibleWithStandardLibrary
 
 type Handler struct {
-	svc *usecase.Service
+	svc          *usecase.Service
+	buyerFeeRate float64 // Rate to inflate prices for buyers (e.g., 0.12 = 12%)
 }
 
-func New(svc *usecase.Service) *Handler {
-	return &Handler{svc: svc}
+func New(svc *usecase.Service, buyerFeeRate float64) *Handler {
+	return &Handler{svc: svc, buyerFeeRate: buyerFeeRate}
 }
 
 // Register godoc

backend/internal/http/handler/handler_test.go

@@ -312,7 +312,7 @@ func newTestHandler() *Handler {
 	repo := NewMockRepository()
 	gateway := &MockPaymentGateway{}
 	svc := usecase.NewService(repo, gateway, 0.05, "test-secret", time.Hour, "test-pepper")
-	return New(svc)
+	return New(svc, 0.12) // 12% buyer fee rate for testing
 }
 
 func TestListProducts(t *testing.T) {
@@ -398,7 +398,7 @@ func TestAdminLogin_Success(t *testing.T) {
 	repo := NewMockRepository()
 	gateway := &MockPaymentGateway{}
 	svc := usecase.NewService(repo, gateway, 0.05, "test-secret", time.Hour, "test-pepper")
-	h := New(svc)
+	h := New(svc, 0.12)
 
 	// Create admin user through service (which hashes password)
 	companyID, _ := uuid.NewV7()
@@ -442,7 +442,7 @@ func TestAdminLogin_WrongPassword(t *testing.T) {
 	repo := NewMockRepository()
 	gateway := &MockPaymentGateway{}
 	svc := usecase.NewService(repo, gateway, 0.05, "test-secret", time.Hour, "test-pepper")
-	h := New(svc)
+	h := New(svc, 0.12)
 
 	// Create admin user
 	companyID, _ := uuid.NewV7()

backend/internal/http/handler/product_handler.go

@@ -142,6 +142,17 @@ func (h *Handler) SearchProducts(w http.ResponseWriter, r *http.Request) {
 		writeError(w, http.StatusInternalServerError, err)
 		return
 	}
+
+	// Apply invisible buyer fee: inflate prices by buyerFeeRate (e.g., 12%)
+	// The buyer sees inflated prices, but the DB stores the original seller price
+	if h.buyerFeeRate > 0 {
+		for i := range result.Products {
+			originalPrice := result.Products[i].PriceCents
+			inflatedPrice := int64(float64(originalPrice) * (1 + h.buyerFeeRate))
+			result.Products[i].PriceCents = inflatedPrice
+		}
+	}
+
 	writeJSON(w, http.StatusOK, result)
 }
 

backend/internal/server/server.go

@@ -37,7 +37,7 @@ func New(cfg config.Config) (*Server, error) {
 	repo := postgres.New(db)
 	gateway := payments.NewMercadoPagoGateway(cfg.MercadoPagoBaseURL, cfg.MarketplaceCommission)
 	svc := usecase.NewService(repo, gateway, cfg.MarketplaceCommission, cfg.JWTSecret, cfg.JWTExpiresIn, cfg.PasswordPepper)
-	h := handler.New(svc)
+	h := handler.New(svc, cfg.BuyerFeeRate)
 
 	mux := http.NewServeMux()