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.
429 from the caption endpoint means the IP is rate-limited (retry later),
not that the video has no captions. Distinguishing it from SourceNone is the
prerequisite for the runner's backoff/retry logic.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Pure domain types matching docs/data-model.md: User, Subscription, Video,
Transcript, Summary, plus Provider and TranscriptSource enums. Stdlib-only,
no outward dependencies — the innermost layer. Video carries user_id per the
per-user-isolation decision (no global dedup).