fix: clean up stale active sessions on startup

After an unclean shutdown, sessions could be left with disconnected_at
NULL, appearing permanently active. Add CloseActiveSessions to the Store
interface and call it at startup to close any leftover sessions.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/storage/sqlite.go

@@ -351,6 +351,16 @@ func (s *SQLiteStore) GetRecentSessions(ctx context.Context, limit int, activeOn
 	return sessions, rows.Err()
 }
 
+func (s *SQLiteStore) CloseActiveSessions(ctx context.Context, disconnectedAt time.Time) (int64, error) {
+	res, err := s.db.ExecContext(ctx, `
+		UPDATE sessions SET disconnected_at = ? WHERE disconnected_at IS NULL`,
+		disconnectedAt.UTC().Format(time.RFC3339))
+	if err != nil {
+		return 0, fmt.Errorf("closing active sessions: %w", err)
+	}
+	return res.RowsAffected()
+}
+
 func (s *SQLiteStore) Close() error {
 	return s.db.Close()
 }