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/store.go

@@ -108,6 +108,11 @@ type Store interface {
 	// GetSessionEvents returns all events for a session ordered by id.
 	GetSessionEvents(ctx context.Context, sessionID string) ([]SessionEvent, error)
 
+	// CloseActiveSessions sets disconnected_at for all sessions that are
+	// still marked as active. This should be called at startup to clean up
+	// sessions left over from a previous unclean shutdown.
+	CloseActiveSessions(ctx context.Context, disconnectedAt time.Time) (int64, error)
+
 	// Close releases any resources held by the store.
 	Close() error
 }