Fix MXF downscale and refresh upload OpenAPI spec

[bjay-wk]

Summary

• fix half-rate output when downscaling interlaced MXF sources
• refresh the CLI public OpenAPI spec from the backend filtered spec
• remove stale preprocess payload fields no longer present in the backend schema
• keep original MXF ffprobe metadata attached to upload requests during local downscale

Validation

• cargo check --all-targets
• cargo test --all-targets