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/user/errors.go

@@ -1,7 +1,37 @@
 package user
 
-import "errors"
+import (
+	"errors"
 
-var (
-	ErrUserNotFound = errors.New("user not found")
+	orm "tm/pkg/mongo"
 )
+
+var ErrUserNotFound = errors.New("user not found")
+
+func mapUserWriteError(err error) error {
+	if err == nil || !orm.IsDuplicateKeyError(err) {
+		return err
+	}
+	switch {
+	case orm.DuplicateKeyMatchesField(err, "phone"):
+		return errors.New("phone number already exists")
+	case orm.DuplicateKeyMatchesField(err, "email"):
+		return errors.New("email already exists")
+	case orm.DuplicateKeyMatchesField(err, "username"):
+		return errors.New("username already exists")
+	default:
+		return err
+	}
+}
+
+func isUserConflictError(err error) bool {
+	if err == nil {
+		return false
+	}
+	switch err.Error() {
+	case "phone number already exists", "email already exists", "username already exists":
+		return true
+	default:
+		return false
+	}
+}