fix(user,customer): return 409 on phone duplicate-key races

Map E11000 from the per-collection phone index to friendly conflict
errors and skip redundant GetByPhone when the phone is unchanged.

Not blocking: IAT test DB may need a one-off dedupe for index build;
phone uniqueness is per collection (users vs customers), not global.

internal/customer/errors.go

@@ -1,7 +1,42 @@
 package customer
 
-import "errors"
+import (
+	"errors"
 
-var (
-	ErrCustomerNotFound = errors.New("customer not found")
+	orm "tm/pkg/mongo"
 )
+
+var ErrCustomerNotFound = errors.New("customer not found")
+
+func mapCustomerWriteError(err error) error {
+	if err == nil || !orm.IsDuplicateKeyError(err) {
+		return err
+	}
+	switch {
+	case orm.DuplicateKeyMatchesField(err, "phone"):
+		return errors.New("customer with this phone number already exists")
+	case orm.DuplicateKeyMatchesField(err, "email"):
+		return errors.New("customer with this email already exists")
+	case orm.DuplicateKeyMatchesField(err, "username"):
+		return errors.New("customer with this username already exists")
+	default:
+		return err
+	}
+}
+
+func isCustomerConflictError(err error) bool {
+	if err == nil {
+		return false
+	}
+	switch err.Error() {
+	case "customer with this phone number already exists",
+		"customer with this email already exists",
+		"customer with this username already exists",
+		"company with this name already exists",
+		"company with this registration number already exists",
+		"company with this tax ID already exists":
+		return true
+	default:
+		return false
+	}
+}