fix(auth): cleanup orphan OIDC/MFA rows on user delete (#1285)
Three User-FK tables (user_oidc_links, user_totp, user_otp_codes)
declare ON DELETE CASCADE in their models, but SQLite ships with
PRAGMA foreign_keys=OFF (the project's existing pattern, mirrored
for APIKey in PR #1182). Without explicit DELETEs, deleting a user
on SQLite leaves orphan rows behind:
