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

cmd/av1dae/main.go

@@ -192,6 +192,7 @@ func processFile(ctx context.Context, inputPath string, cfg *types.Config, enc *
 		MediaType:    mediaType,
 		CRF:          crf,
 		Preset:       preset,
+		LP:           cfg.Encoding.LP,
 		DeleteOrigin: deleteOrigin,
 	}