Support batch account creation via file import (username/password)

[nighca]

Background

As part of account admin UI optimization (parent issue #3273), operators need a faster way to provision many test or internal accounts.

What this issue is about

Support batch account creation (username/password login accounts) by importing from a file in the account admin UI.

Plan

• define supported import file format (e.g. CSV) and validation rules
• provide admin UI flow for file upload/import preview and execution
• implement backend/API handling for batch creation with clear success/failure reporting

Additional context

• Parent: Optimize account admin UI for user operations and automation tokens #3273
• Related PR baseline: Add Account admin UI #3267