Fix integration test cleanup to delete orphaned sources and flu* users

- Add flu* pattern to user cleanup regex (matches users created by
  feed-generator-execution-tests.ts)
- Delete sources linked to test users via source_id before deleting users
  (these sources have creator_id=0 from create_user function)

Closes #119

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>