The append dedup guard was check-then-insert: two concurrent appends of the
same document both pass exists() and both insert. The partial unique index
on (geschichte_id, document_id) WHERE document_id IS NOT NULL closes the
race; append saveAndFlush-es and maps the DataIntegrityViolationException to
the same 409 JOURNEY_DOCUMENT_ALREADY_ADDED as the friendly pre-check. The
CHECK on note length pins the 2000-char contract in the schema, mirroring
chk_text_length on transcription_blocks.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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