feat(api): GET /dictionaries/{name}/snapshots — record-change buckets (backend #2)

Backend #2 из pending-endpoints brief. Frontend TimeTravelModal использует
этот endpoint для slider marks — каждая точка = bucket где минимум одна
запись validFrom попадает.

GET /api/v1/dictionaries/{name}/snapshots?from=...&to=...&granularity=day

Implementation:
- DictionaryRecordRepository.findSnapshotBuckets — native SQL date_trunc
  GROUP BY bucket, ORDER BY bucket DESC. scope filter через text[] из CSV.
- SnapshotsService — gates dict existence/scope, validates granularity
  (hour/day/week) + window, formats RU label (1 апр / сейчас), reverses
  список чтобы oldest first (UI читает слева-направо).
- DictionaryChangelogController новый endpoint /snapshots.

Errors:
- 404 dictionary_not_found
- 400 invalid_granularity (not in {hour,day,week})
- 400 invalid_window (from >= to)

Frontend impact: TimeTravelModal slider marks переходит с recordTimestamps
derived data на реальный API. ~1h frontend swap.

ordinis-domain/src/main/java/cloud/nstart/terravault/ordinis/domain/record/DictionaryRecordRepository.java

@@ -146,4 +146,36 @@ public interface DictionaryRecordRepository extends JpaRepository<DictionaryReco
       @Param("scopesCsv") String[] scopesCsv,
       @Param("at") OffsetDateTime at,
       @Param("polygonGeoJson") String polygonGeoJson);
+
+  /**
+   * Bucketed list snapshot points: timestamps когда records появлялись
+   * (validFrom) в заданном time window. Используется для TimeTravel
+   * slider marks на admin-UI — каждая mark = "interesting" moment где
+   * минимум одна запись changed.
+   *
+   * <p>Granularity: PostgreSQL date_trunc unit ({@code hour}, {@code day},
+   * {@code week}). Возвращает Object[] = {bucket OffsetDateTime,
+   * changedSinceLast Long}. Caller'а потом мапит на DTO.
+   *
+   * @param scopes — CSV array (passed как text[] для unnest in WHERE), скоупы
+   *                которые caller видит. EMPTY → no records visible (caller
+   *                anonymous и scope filter empty).
+   */
+  @Query(value = """
+      SELECT date_trunc(:granularity, valid_from)::timestamptz AS bucket,
+             count(*) AS changed_since_last
+      FROM dictionary_records
+      WHERE dictionary_id = :dictionaryId
+        AND data_scope = ANY (string_to_array(:scopesCsv, ','))
+        AND valid_from >= :from
+        AND valid_from < :to
+      GROUP BY bucket
+      ORDER BY bucket DESC
+      """, nativeQuery = true)
+  List<Object[]> findSnapshotBuckets(
+      @Param("dictionaryId") UUID dictionaryId,
+      @Param("scopesCsv") String scopesCsv,
+      @Param("from") OffsetDateTime from,
+      @Param("to") OffsetDateTime to,
+      @Param("granularity") String granularity);
 }