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.go

@@ -403,6 +403,17 @@ func (e *Encoder) encodeOpus(ctx context.Context, wavs []string, workDir string)
 	return opusFiles, nil
 }
 
+// svtav1Params builds the -svtav1-params value, appending lp=N (logical
+// processors / encoder thread count) only when lp > 0. lp=0 lets SVT-AV1
+// auto-detect, preserving prior behavior.
+func svtav1Params(lp int) string {
+	p := "film-grain=10:film-grain-denoise=1:scd=1:qm-min=4:qm-max=15:keyint=10s"
+	if lp > 0 {
+		p += fmt.Sprintf(":lp=%d", lp)
+	}
+	return p
+}
+
 func (e *Encoder) encodeVideo(ctx context.Context, input, workDir string, opusFiles []string, job *types.Job, metadata *types.Metadata, interlaced bool, streamLangs []StreamMetadata) error {
 	outFile := filepath.Join(workDir, "output.mkv")
 
@@ -418,7 +429,7 @@ func (e *Encoder) encodeVideo(ctx context.Context, input, workDir string, opusFi
 		}
 	}
 
-	svtParams := fmt.Sprintf("film-grain=10:film-grain-denoise=1:scd=1:qm-min=4:qm-max=15:keyint=10s")
+	svtParams := svtav1Params(job.LP)
 
 	zscaleStr, newWidth, err := e.calculateZscaleWidth(ctx, input, 0)
 	if err != nil {