sequenceDiagram
    actor User
    participant API as API Controller
    participant Mediator as MediatR pipeline<br/>(ValidationBehavior)
    box Handler Layer
        participant UpdateUsernameHandler
        participant UserMgr as UserManager(ApplicationUser)
    end
    box Repository / Cross-slice
        participant UserStore as DapperUserStore<br/>(IUserStore/IUserPasswordStore/IUserEmailStore)
    end
    participant DB as SQL Server

    User->>API: PATCH /api/auth/username {newUsername} (requires Bearer access token)
    activate API
    API->>Mediator: Send(UpdateUsernameCommand(GetAuthenticatedUserId(), newUsername))
    activate Mediator
    Note right of Mediator: ValidationBehavior runs UpdateUsernameValidator (same format rules as registration)

    alt Validation fails
        Mediator->>API: throws FluentValidation.ValidationException
        API->>User: 400 Bad Request
    else Validation succeeds
        Mediator->>UpdateUsernameHandler: Handle(command)
        activate UpdateUsernameHandler

        UpdateUsernameHandler->>UserMgr: FindByIdAsync(userAccountId)
        activate UserMgr
        UserMgr->>UserStore: FindByIdAsync(userAccountId)
        activate UserStore
        UserStore->>DB: SELECT ... FROM UserAccount WHERE UserAccountID = @Id
        activate DB
        DB-->>UserStore: row or null
        deactivate DB
        UserStore-->>UserMgr: ApplicationUser or null
        deactivate UserStore
        UserMgr-->>UpdateUsernameHandler: ApplicationUser or null
        deactivate UserMgr

        alt Account not found
            UpdateUsernameHandler->>Mediator: throw NotFoundException
            Mediator->>API: propagate
            API->>User: 404 Not Found
        else Account found
            UpdateUsernameHandler->>UserMgr: SetUserNameAsync(user, newUsername)
            activate UserMgr
            UserMgr->>UserStore: FindByNameAsync(newUsername)
            Note right of UserMgr: Uniqueness check before the UPDATE
            UserMgr->>UserStore: UpdateAsync(user)
            UserStore->>DB: UPDATE UserAccount SET Username = @UserName, ...
            UserStore-->>UserMgr: IdentityResult
            UserMgr-->>UpdateUsernameHandler: IdentityResult
            deactivate UserMgr

            alt !result.Succeeded
                UpdateUsernameHandler->>Mediator: throw ConflictException (DuplicateUserName → "Username already exists")
                Mediator->>API: propagate
                API->>User: 409 Conflict
            else result.Succeeded
                UpdateUsernameHandler-->>Mediator: UpdateUsernamePayload(user.Id, user.UserName)
                deactivate UpdateUsernameHandler
                Mediator-->>API: UpdateUsernamePayload
                deactivate Mediator
                API->>User: 200 OK {message: "Username updated successfully.", payload: {...}}
            end
        end
    end
    deactivate API