feat(search): add SearchMatchData record for per-document match signals

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

backend/src/test/java/org/raddatz/familienarchiv/dto/SearchMatchDataTest.java

@@ -0,0 +1,44 @@
+package org.raddatz.familienarchiv.dto;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.List;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class SearchMatchDataTest {
+
+    @Test
+    void transcription_snippet_is_nullable() {
+        SearchMatchData data = new SearchMatchData(null, List.of(), false, List.of(), List.of());
+
+        assertThat(data.transcriptionSnippet()).isNull();
+    }
+
+    @Test
+    void non_null_list_fields_are_empty_by_default_in_empty_factory() {
+        SearchMatchData data = SearchMatchData.empty();
+
+        assertThat(data.transcriptionSnippet()).isNull();
+        assertThat(data.titleOffsets()).isEmpty();
+        assertThat(data.matchedReceiverIds()).isEmpty();
+        assertThat(data.matchedTagIds()).isEmpty();
+        assertThat(data.senderMatched()).isFalse();
+    }
+
+    @Test
+    void holds_all_field_values() {
+        MatchOffset offset = new MatchOffset(0, 4);
+        SearchMatchData data = new SearchMatchData(
+                "schreibt dir aus dem Feld",
+                List.of(offset),
+                true,
+                List.of(),
+                List.of()
+        );
+
+        assertThat(data.transcriptionSnippet()).isEqualTo("schreibt dir aus dem Feld");
+        assertThat(data.titleOffsets()).containsExactly(offset);
+        assertThat(data.senderMatched()).isTrue();
+    }
+}