feat(search): partial-word matching via to_tsquery prefix queries

Replace websearch_to_tsquery with a CROSS JOIN LATERAL subquery that
appends :* to each lexeme so prefix matches work (e.g. "furchtb" finds
"furchtbar"). websearch_to_tsquery still handles the safe tokenisation
of user input (stop words, special chars, operators); regexp_replace
then adds :* before to_tsquery re-parses the result.

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

backend/src/test/java/org/raddatz/familienarchiv/repository/DocumentFtsTest.java

@@ -79,6 +79,16 @@ class DocumentFtsTest {
         assertThat(ids).hasSize(1);
     }
 
+    @Test
+    void should_find_document_by_partial_word_prefix() {
+        documentRepository.saveAndFlush(document("Ein furchtbarer Brief"));
+        em.clear();
+
+        List<UUID> ids = documentRepository.findRankedIdsByFts("furchtb");
+
+        assertThat(ids).hasSize(1);
+    }
+
     @Test
     void should_not_find_document_when_term_absent() {
         documentRepository.saveAndFlush(document("Familienfoto"));