Editable submission titles

# Editable submission titles

On submit, the submission title copies from the document title at the moment of submission. Teachers can rename the submission while grading. Students can rename after release.

## Problem

Today, a submission's display name is tied to the document's current title — meaning if the student renames the document after submitting, the submission's name changes retroactively. Teachers occasionally want to clarify a submission's title (e.g., "Macbeth essay - Period 3 batch") and can't.

## Goals (rough)

- Submission gets its own `title` field, copied from the document at submission time.
- Teacher can edit the title while grading (pre-release).
- Student can edit the title after the grade is released.

## Open questions

- [ ] Audit: who has rename rights at each stage?
- [ ] Display: when both document and submission have different titles, which shows where?

## Engineering handoff

Not ready. Spec needs full UX walkthrough and edge cases enumerated.