fix: active sessions query, chart update performance

- Active sessions query now finds truly active sessions (started
  anytime, no session.end ever) instead of only today's sessions
- Use uPlot setData() for live WS updates instead of destroying
  and recreating the chart on every event
- Destroy chart only on window change so it recreates with new scale

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

internal/store/postgres/stats.go

@@ -36,23 +36,21 @@ func (d *DB) GetSummary(ctx context.Context) (*Summary, error) {
 	now := time.Now()
 	midnight := time.Date(now.Year(), now.Month(), now.Day(), 0, 0, 0, 0, now.Location())
 
-	// Active sessions: sessions that started today but have not ended today
+	// Active sessions: sessions with a session.start but no session.end (ever)
 	activeQ := `
 		SELECT COUNT(DISTINCT session_id)
 		FROM events
 		WHERE type = 'session.start'
-		  AND ts >= $1
 		  AND session_id IS NOT NULL
 		  AND session_id NOT IN (
 		      SELECT DISTINCT session_id
 		      FROM events
 		      WHERE type = 'session.end'
-		        AND ts >= $1
 		        AND session_id IS NOT NULL
 		  )
 	`
 	var activeSessions int
-	if err := d.sql.QueryRowContext(ctx, activeQ, midnight).Scan(&activeSessions); err != nil {
+	if err := d.sql.QueryRowContext(ctx, activeQ).Scan(&activeSessions); err != nil {
 		return nil, err
 	}