Multi-submission support

# Multi-submission support

Allow multiple submissions per document.

## Problem

Today, submitting a document is effectively one-shot per document — there's no first-class way for a student to submit again.

## Status note

Largely superseded by [revision-flow](revision-flow.md), which addresses the iteration-loop case (revising after a grade is released) with a more opinionated, chained-submission model.

This file is kept as a stub because there's a separate use case revision flow doesn't cover: a student submitting two distinct, parallel essays for the same assignment (e.g., "submit a draft, get feedback, then submit a final" where draft and final are independent rather than linear revisions). If that use case becomes important, flesh out this spec.

## Goals (rough)

- ...

## Open questions

- [ ] Is the parallel-submission use case real? Confirm with Kevin before designing further.

## Engineering handoff

Not ready. Spec needs to be designed if the parallel-submission case proves out.