Bug: clipboard 500 (deadlock) under parallel authoring load

Copying a block to the clipboard intermittently answers
500 {"error": "The Studio servers encountered an error"}. The CMS log shows:

django.db.utils.OperationalError: (1213, 'Deadlock found when trying to get
lock; try restarting transaction')

from content_staging/views.py:136 → api.py:189 save_xblock_to_user_clipboard
→ models.py:176 UserClipboard.save, on the update_or_create of the row.

Load-dependent and somewhat rare: four deadlocks across an E2E CI run. Each
concurrent copy was made by a different user, so the contention is not two writers
on one clipboard row; the surrounding transaction takes locks in an order another
concurrent CMS write can cross.

Expected: retry the deadlock, as the platform does elsewhere for 1213, or
narrow the transaction.

Start at content_staging/views.py:136, api.py:189, and models.py:176 to trace save_xblock_to_user_clipboard and its update_or_create transaction. Search the platform for existing handling of database error 1213 and compare whether retrying or narrowing the transaction fits this path. Done means concurrent copies no longer return a 500 from deadlocks, with coverage for the affected behavior if the existing test entry point is found.