add: queue total ETA from current encode speed

The queue card now shows an estimated time to clear: current job remaining +
sum(pending source durations) / current speed. Durations come from a
path+mtime-keyed cache that probes each file once in the background (ffprobe
format=duration, header-only), so the 1s poll never re-probes; entries are
pruned to the live queue. No speed (idle/held) -> no estimate; files still
being probed mark it partial (shown as '~Xh+').

Closes #9

cmd/av1dae/main.go

@@ -113,7 +113,12 @@ func main() {
 	}
 
 	if addr := *cfg.HTTPAddr; addr != "" {
-		srv := &http.Server{Addr: addr, Handler: server.New(tracker, log, store, controls, cfg.Paths.Input, cfg.Paths.Failed).Handler()}
+		probeDuration := func(p string) (float64, error) {
+			pctx, pcancel := context.WithTimeout(context.Background(), 10*time.Second)
+			defer pcancel()
+			return enc.GetDuration(pctx, p)
+		}
+		srv := &http.Server{Addr: addr, Handler: server.New(tracker, log, store, controls, probeDuration, cfg.Paths.Input, cfg.Paths.Failed).Handler()}
 		go func() {
 			log.Info(fmt.Sprintf("Status server listening on %s", addr))
 			if err := srv.ListenAndServe(); err != nil && err != http.ErrServerClosed {