Hotfix: make news audience migration non-atomic

[TThanos3000]

Summary

• Makes existing production migration news.0010_news_audience_newscomment non-atomic by adding atomic = False.
• Adds regression coverage for the migration atomicity contract and a PostgreSQL-only replay scenario for applying 0010 after existing news_news rows are updated.

Why

Production deploy for ref 9075dc9 failed during news.0010_news_audience_newscomment with PostgreSQL cannot ALTER TABLE news_news because it has pending trigger events. The migration combines schema changes, a RunPython update on news_news, and AddConstraint in one atomic transaction. atomic = False removes that wrapping transaction; the RunPython operation does not pass atomic=True, so it does not reintroduce one.

Scope

• No changes to news/models.py — master already has the matching schema.
• No changes to the news attachment hotfix Hotfix: fix news attachment link validation #698.
• No workflow, Docker, nginx, Celery, dev, React, or frontend changes.

Checks

• DEBUG=True python manage.py makemigrations --check --dry-run — passed, no changes detected.
• DEBUG=True python manage.py test news.tests.test_news_migration_0010 --verbosity=2 — passed: 2 OK, 1 PostgreSQL-only regression skipped locally on sqlite.
• DEBUG=True python manage.py test news --verbosity=2 — passed: 37 OK, 1 PostgreSQL-only regression skipped locally on sqlite.
• python -m flake8 news/migrations/0010_news_audience_newscomment.py news/tests/test_news_migration_0010.py — passed.
• python -m black --diff news/migrations/0010_news_audience_newscomment.py news/tests/test_news_migration_0010.py — passed: 2 files would be left unchanged.

Note: local black --check/black process hung in this Windows shell after formatting, so final format verification was done with black --diff, which exited successfully and reported no changes needed.