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/web/handlers.go

@@ -34,6 +34,9 @@ type Store interface {
 	DeleteConnection(ctx context.Context, userID, provider string) error
 	DeleteUser(ctx context.Context, userID string) error
 	DisplayName(ctx context.Context, userID string) (string, error)
+	// ListChannelErrors returns channels that returned HTTP 404 (deleted/private) on
+	// the most recent discovery pass, shown on the account page as a warning.
+	ListChannelErrors(ctx context.Context, userID string) ([]store.ChannelError, error)
 
 	// StampLogin records (throttled, one row per user per day) that the resolved
 	// user was active on this request — the read-side Stage-0 usage signal the