feat(account): send display name in account state to Core

The account state push carries displayName. It is published when the
name changes. Contract test updated.

readme.md

@@ -350,9 +350,9 @@ nor a database connection waits for Core; a failed call only warns.
 sequenceDiagram
     participant A as Auth
     participant C as Core
-    A->>A: link / unlink / block / unblock (commit)
-    A->>C: POST /internal/accounts/states {userId, steamId, blocked}
-    C->>C: blocked_user, Steam ID in user_steam_sync
+    A->>A: link / unlink / block / unblock / name change (commit)
+    A->>C: POST /internal/accounts/states {userId, steamId, displayName, blocked}
+    C->>C: blocked_user, Steam ID in user_steam_sync, display name in account_name
     Note over A,C: nightly 04:15
     A->>C: PUT /internal/accounts/states (every user)
 ```