feat(admin-invites): show registered usernames per invite code, track invited_by_code_id on User

app/routers/web.py

@@ -153,7 +153,7 @@ def register_submit(
             "register.html", {"request": request, "error": "用户名已存在"}, status_code=400
         )
     # Create user
-    new_user = User(username=username, password_hash=hash_password(password))
+    new_user = User(username=username, password_hash=hash_password(password), invited_by_code_id=code_obj.id)
     db.add(new_user)
     db.flush()
     # Update invite code usage