docs(document): explain the DensityFilters->SearchFilters bridge (#683)

Clarify at loadFilteredDates why the density path constructs a SearchFilters:
the two filter records are kept separate (density has no date/undated fields),
so it adapts here to reuse buildSearchSpec. Raised in the #702 review.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -167,6 +167,9 @@ public class DocumentService {
     /** Loads matching documents and projects to non-null {@link LocalDate}s. */
     private List<LocalDate> loadFilteredDates(DensityFilters filters, List<UUID> ftsIds) {
         boolean hasFts = ftsIds != null;
+        // Density and search keep separate filter records (DensityFilters has no
+        // date/undated fields); adapt to SearchFilters here to reuse buildSearchSpec.
+        // Date bounds stay null and undated=false — the density path never filters by date.
         SearchFilters searchFilters = new SearchFilters(
                 filters.text(), null, null, filters.sender(), filters.receiver(),
                 filters.tags(), filters.tagQ(), filters.status(), filters.tagOperator(), false);