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/adapters/store/channel_errors.go

@@ -0,0 +1,59 @@
+package store
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/jackc/pgx/v5"
+)
+
+// ChannelError is a channel that returned HTTP 404 on a discovery pass.
+type ChannelError struct {
+	ChannelID   string
+	ChannelName string
+	FirstSeen   time.Time
+	LastSeen    time.Time
+}
+
+// UpsertChannelError records (or refreshes) a 404 channel for the current user.
+// Called by the runner inside a withUser scope; RLS guards user isolation.
+func (s *Store) UpsertChannelError(ctx context.Context, userID, channelID, channelName string) error {
+	return s.withUser(ctx, userID, func(tx pgx.Tx) error {
+		_, err := tx.Exec(ctx, `
+			INSERT INTO channel_errors (user_id, channel_id, channel_name)
+			VALUES ($1, $2, $3)
+			ON CONFLICT (user_id, channel_id)
+			DO UPDATE SET channel_name = EXCLUDED.channel_name, last_seen = now()`,
+			userID, channelID, channelName)
+		if err != nil {
+			return fmt.Errorf("store: upsert channel error: %w", err)
+		}
+		return nil
+	})
+}
+
+// ListChannelErrors returns all 404-flagged channels for the user, newest first.
+func (s *Store) ListChannelErrors(ctx context.Context, userID string) ([]ChannelError, error) {
+	var out []ChannelError
+	err := s.withUser(ctx, userID, func(tx pgx.Tx) error {
+		rows, err := tx.Query(ctx, `
+			SELECT channel_id, channel_name, first_seen, last_seen
+			FROM channel_errors
+			WHERE user_id = $1
+			ORDER BY last_seen DESC`, userID)
+		if err != nil {
+			return fmt.Errorf("store: list channel errors: %w", err)
+		}
+		defer rows.Close()
+		for rows.Next() {
+			var ce ChannelError
+			if err := rows.Scan(&ce.ChannelID, &ce.ChannelName, &ce.FirstSeen, &ce.LastSeen); err != nil {
+				return fmt.Errorf("store: scan channel error: %w", err)
+			}
+			out = append(out, ce)
+		}
+		return rows.Err()
+	})
+	return out, err
+}