`SourceStar` is queried via `one_or_none()` without a uniqueness constraint

[cfm]

The Source model assumes that its star attribute will return either zero or one SourceStar instance:

securedrop/securedrop/models.py

Line 93 in 33d51db

star = relationship("SourceStar", uselist=False, backref="source")

But SourceStar.source_id has no uniqueness constraint and is implicitly many-to-one:

securedrop/securedrop/models.py

Lines 465 to 468 in 33d51db

	class SourceStar(db.Model):
	__tablename__ = "source_stars"
	id = Column("id", Integer, primary_key=True)
	source_id = Column("source_id", Integer, ForeignKey("sources.id"))

Two "star source" operations for source s could race and each result in a separate SourceStar(source_id=s.id) record, which would cause subsequent one_or_none() queries to raise MultipleResultsFound.

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Summarized from findings by Claude while investigating other data races. It's not obvious to me from GitHub history why SourceStar is a separate model rather than a Boolean attribute on Source itself, but that's a bigger refactoring than just making sure there is exactly either zero or one of them for a given source.