YouTube channels that 404 on playlist discovery (deleted/private) are now:
1. Wrapped in domain.ErrChannelUnavailable by the YouTube adapter (instead of
a generic error), so the runner can identify them without string-matching.
2. Stored per-user in channel_errors (migration 013, RLS-guarded) via runner's
new UpsertChannelError path — removed from the generic Errors counter,
counted separately as ChannelUnavailable.
3. Shown on the account page under "Unavailable channels" with name, chip-warn
badge, and first-seen date, so users know why some subscribed channels
produce no videos.
