Add 422 status for invalid email domain

src/user.ts

@@ -16,6 +16,7 @@ export type UserApiErrorCode =
     | 'VALIDATION'
     | 'AUTH'
     | 'NOT_FOUND'
+    | 'UNPROCESSABLE_ENTITY'
     | 'RATE_LIMIT'
     | 'SERVER';
 
@@ -23,6 +24,7 @@ const ERROR_DESCRIPTIONS: Record<UserApiErrorCode, string> = {
     VALIDATION:  'The request was invalid or contained malformed data',
     AUTH:        'Authentication failed — missing or invalid API key',
     NOT_FOUND:   'The requested user account was not found',
+    UNPROCESSABLE_ENTITY: 'The registration email is not from a permitted domain',
     RATE_LIMIT:  'Rate limit exceeded — too many requests, try again shortly',
     SERVER:      'The server encountered an internal error',
 };
@@ -69,6 +71,7 @@ function statusToCode(status: number): UserApiErrorCode {
         case 401:
         case 403:  return 'AUTH';
         case 404:  return 'NOT_FOUND';
+        case 422:  return 'UNPROCESSABLE_ENTITY';
         case 429:  return 'RATE_LIMIT';
         default:   return 'SERVER';
     }