feat(fts): add search_vector column, GIN index, DB triggers, and FTS repository method (V34)

- V34 migration: adds search_vector tsvector column with GIN index
- BEFORE INSERT/UPDATE trigger on documents rebuilds vector from title (A),
  summary + transcription_blocks.text (B), sender/receiver names (C),
  tag names + location (D) using german FTS config
- AFTER triggers on transcription_blocks, document_receivers, document_tags
  touch the parent document row to re-fire the BEFORE UPDATE trigger
- DocumentRepository.findRankedIdsByFts() native query using websearch_to_tsquery
- DocumentFtsTest: 12 integration tests covering stemming, trigger sync,
  ranking, stop words, malformed input, receiver and tag search

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

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

@@ -81,4 +81,12 @@ public interface DocumentRepository extends JpaRepository<Document, UUID>, JpaSp
             @Param("to") LocalDate to,
             Sort sort);
 
+    @Query(nativeQuery = true, value = """
+            SELECT d.id FROM documents d
+            WHERE d.search_vector @@ websearch_to_tsquery('german', :query)
+            ORDER BY ts_rank(d.search_vector, websearch_to_tsquery('german', :query)) DESC,
+                     d.meta_date DESC NULLS LAST
+            """)
+    List<UUID> findRankedIdsByFts(@Param("query") String query);
+
 }