fix(api): add Submissions.unsubmit() to clear submitted_at with explicit null

[nycomp]

Stack (merge in order): #78 -> #79 -> #80 -> #81 -> #82 -> #85
This is 1 of 6. Base is the branch of the PR below; each PR's diff shows only its own commit, and GitHub retargets it to main when the one below merges. Branches rebuilt on current main (9563443) — content unchanged from the original commits.

Summary

• Adds Submissions.Submission.unsubmit(), which PATCHes {"submitted_at": null} — the only body the server interprets as clearing submitted_at (campus/api/resources/submission.py:60).
• update() cannot express this: its None means "omit the field", so update(submitted_at=None) raised the client's own "At least one field" ValueError. campus-classroom's unsubmit was inexpressible through the client.
• Mirrors the existing submit() action; unsubmit keeps update()'s contract unchanged (pinned by tests).

Tests

tests/unit/test_submissions_unsubmit.py (same mock-client pattern as test_nested_resources.py):

• unsubmit PATCHes {"submitted_at": None} to /api/v1/submissions/<id>/
• update() still raises ValueError with no fields, and still sets a timestamp string

Full suite: 149 passed (146 existing + 3 new).

Fixes #74