feat(model): add title, personType, displayName to Person entity

- Add title (nullable VARCHAR) and personType (enum, default PERSON)
- Make firstName nullable for non-person entities
- Add @Transient getDisplayName() as single source of truth for
  name display, exposed via @Schema(READ_ONLY, REQUIRED)

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

backend/src/test/java/org/raddatz/familienarchiv/model/PersonTest.java

@@ -0,0 +1,53 @@
+package org.raddatz.familienarchiv.model;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class PersonTest {
+
+    @Test
+    void getDisplayName_withAllFields() {
+        Person person = Person.builder()
+                .title("Dr.")
+                .firstName("Walter")
+                .lastName("de Gruyter")
+                .build();
+        assertThat(person.getDisplayName()).isEqualTo("Dr. Walter de Gruyter");
+    }
+
+    @Test
+    void getDisplayName_withoutTitle() {
+        Person person = Person.builder()
+                .firstName("Clara")
+                .lastName("Cram")
+                .build();
+        assertThat(person.getDisplayName()).isEqualTo("Clara Cram");
+    }
+
+    @Test
+    void getDisplayName_withNullFirstName() {
+        Person person = Person.builder()
+                .lastName("Gesellschafter des Verlages")
+                .build();
+        assertThat(person.getDisplayName()).isEqualTo("Gesellschafter des Verlages");
+    }
+
+    @Test
+    void getDisplayName_withTitleAndNullFirstName() {
+        Person person = Person.builder()
+                .title("Tante")
+                .lastName("Molly")
+                .build();
+        assertThat(person.getDisplayName()).isEqualTo("Tante Molly");
+    }
+
+    @Test
+    void personType_defaultsToPerson() {
+        Person person = Person.builder()
+                .firstName("Clara")
+                .lastName("Cram")
+                .build();
+        assertThat(person.getPersonType()).isEqualTo(PersonType.PERSON);
+    }
+}