submit: default validation + hf_username fields on the pending row

submit: stuck-pending sweep on boot (chunk 6)

submit: reject byte-identical re-submits via SHA-256 dedup

submit: background-thread eval, flip row pending -> completed / failed

submit: drop the agree checkbox, meta.json is the only consent gate

submit: drop dead form fields, meta.json is the only source

submit: write pending row + upload zip on successful validation

submit: move handle_submit into its own module + add validation pipeline