Bumps go-pkgz/auth to v2.1.5, which adds avatar.ErrNotFound. deleteMeRequestCtrl's
avatar removal was best-effort (log and continue on any error) because before the
sentinel there was no portable way to tell an already-removed avatar from a genuine
failure. It now tolerates only errors.Is(err, avatar.ErrNotFound) - keeping the
repeated-request idempotency - and surfaces any other store failure as 500.
Add AUTH_MICROSOFT_TENANT env var to allow configuring the Azure AD
tenant for single-tenant Entra ID applications, which cannot use the
default /common endpoint.
Depends on go-pkgz/auth#266
Closes#1998