The index was added in anticipation of a SQL GROUP BY aggregation,
but DocumentService.getDensity aggregates in memory via
findAll(spec).stream(). The index is never touched by the current
query plan. Per Markus's round-2 review: drop the unused migration
to avoid mismatched rationale-vs-implementation debt. Revisit when
the archive crosses 50k rows (TODO already in getDensity Javadoc).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>