Also adds vitest.config.js with fileParallelism: false. Every DB-backed
test file wipes and reseeds the shared conversion_jobs table in
beforeEach; running test files in parallel (Vitest's default) let one
file's DELETE race another file's just-inserted row against the same
live MariaDB instance, causing intermittent cross-file failures.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
