Add @ExceptionHandler(DataIntegrityViolationException) returning 400
VALIDATION_ERROR with a fixed constant message, so any integrity violation
that slips past the upstream guards (a future constraint, or the import
path) becomes a clean 400 instead of a 500 + Sentry alert (AC9).
Deliberately generic — it does not inspect which constraint failed. Never
echoes ex.getMessage() (constraint name + SQL, CWE-209), logs at WARN
without passing the exception (would re-leak the SQL to Loki), and does not
call Sentry.captureException.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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