Identify account registration method (OTP/Magic-Google/Magic-Apple) in Mobile BFF Account Deletion Slack Integration

## Description

Modify the Mobile BFF <> Slack integration so that Account Deletion request messages identify the account's registration method (OTP, Magic-Google, Magic-Apple). This allows the team to process deletion requests accordingly based on account type.

## Context

Currently, Account Deletion Slack notifications don't surface how the account was registered, making it harder to route/process requests correctly (e.g., Magic and OTP accounts may hold wearables/credits and need different handling than pure Guest accounts).

## Related

- #2338 — Automate Account Deletion for Magic and OTP Users (this issue is a smaller, immediate step toward that broader automation goal)
- #2335 — Guest User: Wire Automatic Account Deletion for Non-Upgraded Guests

---
Requested by Rodri via Slack