feat(persons): add notes field to person profile (issue #23)

V5 Flyway migration adds TEXT notes column; Person entity, service, and
controller updated to persist notes. Frontend edit form adds textarea and
view mode renders the notes section. Backed by 2 new service unit tests
(persist + blank clears).

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

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

@@ -83,6 +83,32 @@ class PersonServiceTest {
         verify(personRepository).findByAliasIgnoreCase("Clara Cram");
     }
 
+    // ─── updatePerson (notes) ────────────────────────────────────────────────
+
+    @Test
+    void updatePerson_persistsNotes() {
+        UUID id = UUID.randomUUID();
+        Person person = Person.builder().id(id).firstName("Anna").lastName("Alt").build();
+        when(personRepository.findById(id)).thenReturn(Optional.of(person));
+        when(personRepository.save(any())).thenAnswer(inv -> inv.getArgument(0));
+
+        Person result = personService.updatePerson(id, "Anna", "Alt", null, "Some notes here.");
+
+        assertThat(result.getNotes()).isEqualTo("Some notes here.");
+    }
+
+    @Test
+    void updatePerson_clearsNotes_whenBlank() {
+        UUID id = UUID.randomUUID();
+        Person person = Person.builder().id(id).firstName("Anna").lastName("Alt").notes("old notes").build();
+        when(personRepository.findById(id)).thenReturn(Optional.of(person));
+        when(personRepository.save(any())).thenAnswer(inv -> inv.getArgument(0));
+
+        Person result = personService.updatePerson(id, "Anna", "Alt", null, "   ");
+
+        assertThat(result.getNotes()).isNull();
+    }
+
     // ─── mergePersons ─────────────────────────────────────────────────────────
 
     @Test