Delete the current user's account.

Sole owners must transfer ownership or contact support before account deletion can proceed. This prevents orphaning organizations; org auto-delete is intentionally not part of this endpoint.

What gets deleted:

• User's memberships in all orgs
• User's role grants
• User's API keys
• Audit trail references (nullified, not deleted)
• User profile (public.users)
• Auth account (auth.users)

What gets preserved (nullified):

• Invites sent by user (invited_by_user_id → NULL)
• Role grants created by user (granted_by → NULL)
• API keys created by user (created_by → NULL)

Returns: 204 No Content on success

Raises: 400: User is sole owner of one or more orgs

Example error response: { "detail": "Cannot delete account while you are the sole owner of these organizations. Transfer ownership or contact support: Acme Corp (org_abc)" }