fix(audit): add blockId to TEXT_SAVED audit payload

Required for dashboard Pulse stat 2 (COUNT DISTINCT blockId).
Without it, two saves on different blocks on the same page
were indistinguishable.

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

backend/src/test/java/org/raddatz/familienarchiv/service/TranscriptionServiceTest.java

@@ -487,6 +487,7 @@ class TranscriptionServiceTest {
                 org.mockito.ArgumentMatchers.eq(docId),
                 payloadCaptor.capture());
         assertThat(payloadCaptor.getValue()).containsEntry("pageNumber", 3);
+        assertThat(payloadCaptor.getValue()).containsEntry("blockId", blockId.toString());
     }
 
     @Test