feat(store,runner,web): channel unavailability notice (migration 013)

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.

internal/runner/runner_test.go

@@ -91,6 +91,8 @@ func (f *fakeStore) RateLimitedVideoIDs(_ context.Context, _ string) (map[string
 	return cp, nil
 }
 
+func (f *fakeStore) UpsertChannelError(_ context.Context, _, _, _ string) error { return nil }
+
 func (f *fakeStore) SetTranscriptStatus(_ context.Context, _, videoID, status string) error {
 	if f.statuses == nil {
 		f.statuses = map[string]string{}