refactor: address PR review concerns

- Remove Architekt from WORD_PREFIXES (classifier handles it)
- Use Objects.equals for null-safe firstName/lastName comparison
- Remove unused trimmed variable in PersonTypeClassifier
- Fix containsWord to loop through all occurrences (finds
  "Eltern" in "Nachbareltern Eltern")
- Extract DisplayNameFormatter utility shared by Person and
  PersonSummaryDTO to eliminate display logic duplication

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

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

@@ -86,4 +86,9 @@ class PersonTypeClassifierTest {
     void classify_caseInsensitive() {
         assertThat(PersonTypeClassifier.classify("firma auschrath")).isEqualTo(PersonType.INSTITUTION);
     }
+
+    @Test
+    void classify_containsWord_findsSecondOccurrence() {
+        assertThat(PersonTypeClassifier.classify("Nachbareltern Eltern")).isEqualTo(PersonType.GROUP);
+    }
 }