chore(documents): drop V61 timeline density index migration (#385)

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>

backend/src/main/resources/db/migration/V61__add_document_date_index.sql

@@ -1,9 +0,0 @@
--- Index on documents.meta_date for the timeline density aggregation (issue #385).
--- The new GET /api/documents/density endpoint does GROUP BY date_trunc('month', meta_date)
--- across the full table; an index keeps the aggregation cheap as the archive grows.
--- Cheap to add at any size and removes the future-investigation tax.
---
--- Note: the entity field is `documentDate` but it's mapped to the PostgreSQL column
--- `meta_date` (see Document.java @Column(name = "meta_date")).
-
-CREATE INDEX IF NOT EXISTS idx_documents_meta_date ON documents (meta_date);