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

internal/server/server_test.go

@@ -0,0 +1,62 @@
+package server
+
+import (
+	"testing"
+	"time"
+)
+
+func TestQueueETASeconds(t *testing.T) {
+	// No speed → can't extrapolate.
+	if got := queueETASeconds(100, 3600, 0); got != 0 {
+		t.Errorf("speed 0: got %d, want 0", got)
+	}
+	// current remaining 600s + 1800s of queued source at 0.5x (=3600s) = 4200s.
+	if got := queueETASeconds(600, 1800, 0.5); got != 4200 {
+		t.Errorf("got %d, want 4200", got)
+	}
+	// At 2x, 1800s of source encodes in 900s; + 600 remaining = 1500.
+	if got := queueETASeconds(600, 1800, 2); got != 1500 {
+		t.Errorf("got %d, want 1500", got)
+	}
+	// Empty queue → just the current job's remaining.
+	if got := queueETASeconds(600, 0, 1); got != 600 {
+		t.Errorf("got %d, want 600", got)
+	}
+}
+
+// waitCached polls Get (the probe resolves on a background goroutine).
+func waitCached(c *durationCache, p string, mt time.Time) (float64, bool) {
+	for i := 0; i < 200; i++ {
+		if sec, ok := c.Get(p, mt); ok {
+			return sec, true
+		}
+		time.Sleep(2 * time.Millisecond)
+	}
+	return 0, false
+}
+
+func TestDurationCache(t *testing.T) {
+	c := newDurationCache(func(p string) (float64, error) { return 42, nil })
+
+	mt := time.Unix(1000, 0)
+	if _, ok := c.Get("/a.mkv", mt); ok {
+		t.Fatal("first Get should miss")
+	}
+	if sec, ok := waitCached(c, "/a.mkv", mt); !ok || sec != 42 {
+		t.Fatalf("after probe: got %v ok=%v, want 42 true", sec, ok)
+	}
+
+	// A changed mtime invalidates the entry.
+	if _, ok := c.Get("/a.mkv", time.Unix(2000, 0)); ok {
+		t.Error("changed mtime should miss")
+	}
+	if _, ok := waitCached(c, "/a.mkv", time.Unix(2000, 0)); !ok {
+		t.Fatal("re-probe under new mtime should cache")
+	}
+
+	// Retain drops entries for paths not in the keep set.
+	c.Retain([]string{"/b.mkv"})
+	if _, ok := c.Get("/a.mkv", time.Unix(2000, 0)); ok {
+		t.Error("Retain should have dropped /a.mkv")
+	}
+}