feat(search): surface summary snippet when summary matched the query

Add a summary_snippet column to findEnrichmentData using ts_headline on
documents.summary, only when the summary's tsvector matches the query.
Expose it via SearchMatchData.summarySnippet / summaryOffsets and render
a "Zusammenfassung" / "Summary" / "Resumen" labelled row in the document
list — identical treatment to the transcription snippet row.

Fixes the case where a document appeared in search results with no
visible match explanation (e.g. searching "frucht" found a document
whose summary mentioned "Früchte").

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

backend/src/test/java/org/raddatz/familienarchiv/controller/DocumentControllerTest.java

@@ -125,7 +125,7 @@ class DocumentControllerTest {
                 .status(DocumentStatus.UPLOADED)
                 .build();
         var matchData = new org.raddatz.familienarchiv.dto.SearchMatchData(
-                "Er schrieb einen langen Brief", List.of(), false, List.of(), List.of(), List.of());
+                "Er schrieb einen langen Brief", List.of(), false, List.of(), List.of(), List.of(), null, List.of());
         when(documentService.searchDocuments(any(), any(), any(), any(), any(), any(), any(), any(), any(), any()))
                 .thenReturn(DocumentSearchResult.withMatchData(List.of(doc), Map.of(docId, matchData)));
 

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

@@ -36,7 +36,7 @@ class DocumentSearchResultTest {
     @Test
     void withMatchData_exposes_match_data_map() {
         UUID id = UUID.randomUUID();
-        SearchMatchData data = new SearchMatchData("snippet", List.of(), false, List.of(), List.of(), List.of());
+        SearchMatchData data = new SearchMatchData("snippet", List.of(), false, List.of(), List.of(), List.of(), null, List.of());
         DocumentSearchResult result = DocumentSearchResult.withMatchData(List.of(doc(id)), Map.of(id, data));
 
         assertThat(result.matchData()).containsKey(id);

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

@@ -10,7 +10,7 @@ class SearchMatchDataTest {
 
     @Test
     void transcription_snippet_is_nullable() {
-        SearchMatchData data = new SearchMatchData(null, List.of(), false, List.of(), List.of(), List.of());
+        SearchMatchData data = new SearchMatchData(null, List.of(), false, List.of(), List.of(), List.of(), null, List.of());
 
         assertThat(data.transcriptionSnippet()).isNull();
     }
@@ -35,6 +35,8 @@ class SearchMatchDataTest {
                 true,
                 List.of(),
                 List.of(),
+                List.of(),
+                null,
                 List.of()
         );
 
@@ -58,7 +60,9 @@ class SearchMatchDataTest {
                 false,
                 List.of(),
                 List.of(),
-                List.of(offset)
+                List.of(offset),
+                null,
+                List.of()
         );
         assertThat(data.snippetOffsets()).containsExactly(offset);
     }