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/memstore.go

@@ -286,6 +286,21 @@ func (m *MemoryStore) GetRecentSessions(_ context.Context, limit int, activeOnly
 	return sessions, nil
 }
 
+func (m *MemoryStore) CloseActiveSessions(_ context.Context, disconnectedAt time.Time) (int64, error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	var count int64
+	t := disconnectedAt.UTC()
+	for _, s := range m.Sessions {
+		if s.DisconnectedAt == nil {
+			s.DisconnectedAt = &t
+			count++
+		}
+	}
+	return count, nil
+}
+
 func (m *MemoryStore) Close() error {
 	return nil
 }