test(transcription): rename latency floor test to reflect 5s assertion

Method said inUnderTwoSeconds; assertion checks isLessThan(5000L) with message
"5s". Three sources of truth, three different values. Rename aligns method name
with the assertion that was intentionally raised from 2s to 5s in a prior commit.

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

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

@@ -165,7 +165,7 @@ class PersonMentionPropagationListenerTest {
     }
 
     @Test
-    void propagatesAcross200Blocks_inUnderTwoSeconds_latencyFloor() {
+    void propagatesAcross200Blocks_inUnderFiveSeconds_latencyFloor() {
         UUID personId = savedPersonId("Auguste", "Raddatz");
         List<UUID> blockIds = new ArrayList<>();
         for (int i = 0; i < 200; i++) {