fix(user): preserve multi-tenant companyCodes and fix Kratos code parsing

- UpdateUser: Implement 'Preserve & Merge' logic to fetch existing joined
  tenants from Keto and merge them with UI requests, preventing the
  loss of multi-tenant affiliations.
- Keto Sync: Expand the self-healing background job to iterate over all
  companyCodes, ensuring 'members' relations are created for every
  joined tenant (fixes #554).
- AuthHandler: Update extractFirstString to gracefully handle numeric
  JSON types, fixing an issue where Kratos login codes were lost during
  Courier webhook processing.

backend/internal/handler/auth_handler.go

@@ -3767,6 +3767,13 @@ func extractFirstString(data map[string]interface{}, keys ...string) string {
 			if str, ok := val.(string); ok && str != "" {
 				return str
 			}
+			// Handle numeric types by converting to string
+			if num, ok := val.(float64); ok {
+				return fmt.Sprint(num)
+			}
+			if num, ok := val.(int); ok {
+				return fmt.Sprint(num)
+			}
 		}
 	}
 	return ""