Eight TDD tasks covering the units module, the row builder that removes the
matched-id off-by-one, the PocketBase schema additions, the route rewiring,
dual-unit display, the backfill migration for the 57 existing rows, and the
operational steps to stop n8n minting orphan records and recover the two
lost notes from scan_20260915.
Also corrects the spec to name the migration as Python, matching the
existing scripts/ convention.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>