Audit every background job for run-twice safety and add idempotency guards

A payment job ran twice after a worker died before it acknowledged the message, and 37 customers were charged two times. The engineer then spent two weeks checking all 140 Celery jobs in a six-year-old Django app with Claude Code. First, Claude filled in a strict form for each job, with file and line evidence, and had to say "unknown" when it wasn't sure. The engineer wrote the one dedup guard by hand, a unique key in Postgres, and Claude added it to the risky jobs about ten at a time. A test that runs every job twice then found 14 jobs the audit had missed. Claude's blind spot was hidden side effects like Django signals, save() overrides, and database triggers.