feat(API): optional display name on users

features/user_response_field_hygiene.feature

@@ -7,11 +7,11 @@ Feature: API responses never expose a user's secret fields
     credential or a still-valid reset/verification secret
 
   Background:
-    Given the public shape of a user in any API response is exactly six fields: id, username,
-      email, is_verified, created_at and updated_at
-    And password, password_reset_code, verification_code, verification_expiration and
-      password_reset_expiration never appear in any response body, under any key name, regardless
-      of their value or whether they are set
+    Given the public shape of a user in any API response is exactly seven fields: id, username,
+      email, is_verified, created_at, updated_at and display_name
+    And password, password_reset_code, verification_code, verification_expiration,
+      password_reset_expiration and token_version never appear in any response body, under any key
+      name, regardless of their value or whether they are set
 
   Scenario: A successful verification response carries only public fields
     Given a user repository containing user id 7, username "alice", email "alice@example.com",
@@ -21,10 +21,11 @@ Feature: API responses never expose a user's secret fields
     When the client sends POST /v1/auth/verify with body {"code": "abc123"}
     Then the response status is 200
     And the response body is a JSON object with exactly the keys id, username, email,
-      is_verified, created_at and updated_at
+      is_verified, created_at, updated_at and display_name
     And the response body does not contain the keys password, password_reset_code,
-      verification_code, verification_expiration or password_reset_expiration
+      verification_code, verification_expiration, password_reset_expiration or token_version
     And the response body's is_verified is true
+    And the response body's display_name is ""
 
   Scenario: A rejected verification attempt never echoes any user field
     Given a user repository containing user id 7, username "alice", is_verified false, whose