catalog: SAO declare seed_field for seed_surface

asset-engine's seed_surface contract needs reproducibility.seed_field to name
which CatalogField carries the seed so the UI can render a Roll button and the
server can fill empty seeds before persistence. SAO's seed field is type=number,
satisfying CatalogService._validate_seed_field.

ace-step is intentionally not declared here — actual_seeds is type=json (a list)
with an upstream reproducibility gap; the planned fix surfaces resolved seeds via
response header, pending a separate contract.

docs/asset-engine/services.yaml

@@ -767,6 +767,7 @@ services:
       mime: audio/wav
     reproducibility:
       seedable: true
+      seed_field: seed
       deterministic: true
       notes: >
         Identical seed+prompt+steps+cfg = bit-identical bytes. Wrapper serializes