add: SVT-AV1 lp (thread cap) config knob

Add encoding.lp (0 = auto) so a long encode can be capped to N logical
processors instead of pinning every core, as a lighter-weight alternative to
the compose cgroup limit. Threaded config -> types.Job -> -svtav1-params via a
testable svtav1Params helper. Settings-UI surfacing stays with #1.

refs #5

internal/encoder/encoder_test.go

@@ -0,0 +1,23 @@
+package encoder
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestSvtav1Params(t *testing.T) {
+	const base = "film-grain=10:film-grain-denoise=1:scd=1:qm-min=4:qm-max=15:keyint=10s"
+
+	if got := svtav1Params(0); got != base {
+		t.Errorf("lp=0 should not append lp:\n got %q\nwant %q", got, base)
+	}
+	if got := svtav1Params(-1); got != base {
+		t.Errorf("lp<0 should not append lp: got %q", got)
+	}
+	if got := svtav1Params(4); got != base+":lp=4" {
+		t.Errorf("lp=4: got %q, want suffix :lp=4", got)
+	}
+	if got := svtav1Params(4); !strings.HasPrefix(got, base) {
+		t.Errorf("lp set should keep the base params intact: got %q", got)
+	}
+}