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.

pkg/mongo/duplicate.go

@@ -0,0 +1,44 @@
+package mongo
+
+import (
+	"errors"
+	"strings"
+
+	mongodriver "go.mongodb.org/mongo-driver/v2/mongo"
+)
+
+// IsDuplicateKeyError reports whether err is a MongoDB E11000 duplicate key error.
+func IsDuplicateKeyError(err error) bool {
+	if err == nil {
+		return false
+	}
+	if strings.Contains(err.Error(), "E11000") || strings.Contains(err.Error(), "duplicate key") {
+		return true
+	}
+	var we mongodriver.WriteException
+	if errors.As(err, &we) {
+		for _, wErr := range we.WriteErrors {
+			if wErr.Code == 11000 {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+// DuplicateKeyMatchesField reports whether a duplicate key error involves the given field
+// (e.g. "phone" matches index phone_idx or dup key { phone: "..." }).
+func DuplicateKeyMatchesField(err error, field string) bool {
+	if err == nil || field == "" {
+		return false
+	}
+	msg := strings.ToLower(err.Error())
+	field = strings.ToLower(field)
+	if strings.Contains(msg, field+"_idx") {
+		return true
+	}
+	if strings.Contains(msg, "dup key") && strings.Contains(msg, field) {
+		return true
+	}
+	return strings.Contains(msg, `"`+field+`"`) || strings.Contains(msg, field+":")
+}