DeleteUser permanently removes a user and all owned data, scoped via
withUser. The users-row ON DELETE CASCADE reaches videos, transcripts,
summaries → sink_deliveries, video_connections, and the user_identities
map (cascades bypass RLS, so a scoped connection still wipes child rows).
summary_actions carries user_id but has NO FK to users (migration 002),
so it is deleted explicitly in the same scoped transaction. Idempotent.
Tapir-side only (decision 2026-06-03): Dex identity is left untouched;
secrets live in the SecretStore and are removed by the account handler.
DisplayName returns the registered name for the account page.
Test proves deletion removes every row for the target user across all
isolated tables (incl. video_connections AND user_identities) and leaves
another user's rows fully intact.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
