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>

cmd/oubliette/main.go

@@ -65,6 +65,13 @@ func run() error {
 	}
 	defer store.Close()
 
+	// Clean up sessions left active by a previous unclean shutdown.
+	if n, err := store.CloseActiveSessions(context.Background(), time.Now()); err != nil {
+		return fmt.Errorf("close stale sessions: %w", err)
+	} else if n > 0 {
+		logger.Info("closed stale sessions from previous run", "count", n)
+	}
+
 	ctx, cancel := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
 	defer cancel()