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>

backend/src/main/java/org/raddatz/familienarchiv/document/DocumentDateRangeProjection.java

@@ -0,0 +1,13 @@
+package org.raddatz.familienarchiv.document;
+
+import java.time.LocalDate;
+
+/**
+ * Spring Data projection for the document_date min/max query backing the timeline
+ * density widget (issue #385). Column aliases in the native SQL must match these
+ * getter names exactly. Both values are {@code null} when the documents table is empty.
+ */
+public interface DocumentDateRangeProjection {
+    LocalDate getMinDate();
+    LocalDate getMaxDate();
+}

backend/src/main/java/org/raddatz/familienarchiv/document/DocumentRepository.java

@@ -240,4 +240,28 @@ public interface DocumentRepository extends JpaRepository<Document, UUID>, JpaSp
             """)
     TranscriptionWeeklyStatsProjection findWeeklyStats();
 
+    /**
+     * Document count grouped by calendar month for the timeline density widget (issue #385).
+     * Each row is {@code Object[]{ String yearMonth, long count }}.
+     * Months without documents are not present — the frontend fills gaps from minDate/maxDate.
+     */
+    @Query(nativeQuery = true, value = """
+            SELECT TO_CHAR(DATE_TRUNC('month', meta_date), 'YYYY-MM') AS month,
+                   COUNT(*)                                          AS doc_count
+              FROM documents
+             WHERE meta_date IS NOT NULL
+               AND (CAST(:from AS date) IS NULL OR meta_date >= :from)
+               AND (CAST(:to   AS date) IS NULL OR meta_date <= :to)
+             GROUP BY 1
+             ORDER BY 1
+            """)
+    List<Object[]> findDensityByMonth(@Param("from") LocalDate from, @Param("to") LocalDate to);
+
+    /**
+     * Earliest and latest {@code meta_date} across all documents. Both getters return
+     * {@code null} when the table holds no documents.
+     */
+    @Query(nativeQuery = true, value = "SELECT MIN(meta_date) AS minDate, MAX(meta_date) AS maxDate FROM documents")
+    DocumentDateRangeProjection findMinMaxDocumentDate();
+
 }