Add DELETE /api/me/account: permanently erases the signed-in account and all its
personal data — OAuth tokens, subscriptions, tags, video states, playlists,
notifications all cascade on the users row (FK ON DELETE CASCADE); quota-audit
events are kept but anonymised (SET NULL); the email's invite/whitelist row is
removed too. Guards: the shared demo account can't be deleted, and the last
remaining admin can't delete itself. Frontend: a Danger zone in Settings -> Account
(non-demo) with a danger-confirm; on success the session clears and the app lands
on the welcome page. EN/HU/DE. Verified via curl: self-delete + session clear +
demo 403.
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