feat(documents): add density and date-range repository queries (#385)
Native SQL aggregations backing GET /api/documents/density:
- findDensityByMonth groups documents by truncated meta_date with optional
from/to bounds (frontend fills zero-count gaps).
- findMinMaxDocumentDate returns the earliest/latest meta_date via projection,
null on empty archive.
Covered by DocumentDensityIntegrationTest (Testcontainers PostgreSQL): empty
archive, single+multi-month grouping, from/to bounds, null meta_date exclusion,
min/max edge cases.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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