sequenceDiagram
    actor User
    participant API as API Controller
    participant Mediator as MediatR pipeline<br/>(ValidationBehavior)
    box Handler Layer
        participant DeleteAccountHandler
        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: DELETE /api/auth/account (requires Bearer access token)
    activate API
    API->>Mediator: Send(DeleteAccountCommand(GetAuthenticatedUserId()))
    activate Mediator
    Mediator->>DeleteAccountHandler: Handle(command)
    activate DeleteAccountHandler

    DeleteAccountHandler->>UserMgr: FindByIdAsync(userAccountId)
    activate UserMgr
    UserMgr->>UserStore: FindByIdAsync(userAccountId)
    UserStore-->>UserMgr: ApplicationUser or null
    UserMgr-->>DeleteAccountHandler: ApplicationUser or null
    deactivate UserMgr

    alt Account not found
        DeleteAccountHandler->>Mediator: throw NotFoundException
        Mediator->>API: propagate
        API->>User: 404 Not Found
    else Account found
        DeleteAccountHandler->>UserMgr: DeleteAsync(user)
        activate UserMgr
        UserMgr->>UserStore: DeleteAsync(user)
        activate UserStore
        UserStore->>DB: DELETE FROM UserAccount WHERE UserAccountID = @Id
        Note right of UserStore: UserCredential, UserVerification, and UserProfile cascade on delete<br/>(UserAvatar cascades transitively through UserProfile).<br/>Posts, comments, photos, and follows use ON DELETE NO ACTION<br/>if any reference this account, the DELETE throws a foreign key violation (SQL error 547)

        alt Foreign key violation (dependent records exist)
            UserStore-->>UserMgr: IdentityResult.Failed (AccountHasDependentRecords)
        else Delete succeeds
            UserStore-->>UserMgr: IdentityResult.Success
        end
        deactivate UserStore
        UserMgr-->>DeleteAccountHandler: IdentityResult
        deactivate UserMgr

        alt Account has dependent records
            DeleteAccountHandler->>Mediator: throw ConflictException<br>"Account cannot be deleted while it still has associated posts, comments, photos, or follows."
            Mediator->>API: propagate
            API->>User: 409 Conflict
        else !result.Succeeded (other failure)
            DeleteAccountHandler->>Mediator: throw ConflictException
            Mediator->>API: propagate
            API->>User: 409 Conflict
        else result.Succeeded
            DeleteAccountHandler-->>Mediator: (void)
            deactivate DeleteAccountHandler
            Mediator-->>API: (void)
            deactivate Mediator
            API->>User: 200 OK {message: "Account deleted successfully."}
        end
    end
    deactivate API