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

config.example.yaml

@@ -11,6 +11,9 @@ encoding:
   bluray:
     crf: 29
     preset: 3
+  # SVT-AV1 logical-processor (thread) cap so a long encode can't pin every core.
+  # 0 = auto (use all cores). Applies to every profile.
+  lp: 0
 
 paths:
   input: "./input"