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.go

@@ -38,6 +38,10 @@ type VideoStore interface {
 	// SetTranscriptStatus records the outcome of a transcript attempt: "none",
 	// "rate_limited" (stamps the backoff clock), or "fetched".
 	SetTranscriptStatus(ctx context.Context, userID, videoID, status string) error
+	// UpsertChannelError records a channel that returned HTTP 404 (deleted/private).
+	// Called when NewVideos returns domain.ErrChannelUnavailable; best-effort, errors
+	// are logged and never abort the pass.
+	UpsertChannelError(ctx context.Context, userID, channelID, channelTitle string) error
 }
 
 // Processor runs the core use case for a single video. *usecase.Engine
@@ -94,6 +98,7 @@ type Stats struct {
 	SkippedManual      int // discovered but not queued, in manual mode
 	SkippedRateLimited int // 429'd previously and still inside the backoff window
 	Errors             int
+	ChannelUnavailable int // channels that returned HTTP 404 (deleted/private)
 }
 
 // RunOnce performs a single pass over the user's subscriptions. Per-item errors
@@ -152,8 +157,17 @@ func (r *Runner) RunOnce(ctx context.Context) (Stats, error) {
 	for _, sub := range subs {
 		vids, err := r.src.NewVideos(ctx, sub)
 		if err != nil {
-			errs = append(errs, fmt.Errorf("new videos for %q: %w", sub.ChannelTitle, err))
-			stats.Errors++
+			var unavail *domain.ErrChannelUnavailable
+			if errors.As(err, &unavail) {
+				stats.ChannelUnavailable++
+				r.log.Warn("channel unavailable (playlist 404)", "channel", sub.ChannelTitle, "channel_id", sub.ChannelID)
+				if storeErr := r.store.UpsertChannelError(ctx, r.userID, unavail.ChannelID, unavail.ChannelTitle); storeErr != nil {
+					r.log.Warn("failed to store channel error", "err", storeErr)
+				}
+			} else {
+				errs = append(errs, fmt.Errorf("new videos for %q: %w", sub.ChannelTitle, err))
+				stats.Errors++
+			}
 			continue
 		}
 		for _, v := range vids {
@@ -251,7 +265,7 @@ func (r *Runner) Loop(ctx context.Context, interval time.Duration) error {
 			"candidates", stats.Candidates, "summarized", stats.Summarized,
 			"skipped_seen", stats.SkippedSeen, "skipped_no_text", stats.SkippedNoText,
 			"skipped_manual", stats.SkippedManual, "skipped_rate_limited", stats.SkippedRateLimited,
-			"errors", stats.Errors)
+			"channel_unavailable", stats.ChannelUnavailable, "errors", stats.Errors)
 		if err != nil {
 			r.log.Warn("run pass had errors", "err", err)
 		}