Add persistence layer with Liquibase migration

JPA entity, repository, persistence adapter for events. Liquibase
changelog creates the events table with BIGSERIAL ID and UUID tokens.

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

backend/src/test/java/de/fete/adapter/out/persistence/EventPersistenceAdapterTest.java

@@ -0,0 +1,97 @@
+package de.fete.adapter.out.persistence;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import de.fete.TestcontainersConfig;
+import de.fete.domain.model.Event;
+import de.fete.domain.port.out.EventRepository;
+import java.time.LocalDate;
+import java.time.OffsetDateTime;
+import java.time.ZoneOffset;
+import java.util.Optional;
+import java.util.UUID;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.context.annotation.Import;
+
+@SpringBootTest
+@Import(TestcontainersConfig.class)
+class EventPersistenceAdapterTest {
+
+  @Autowired
+  private EventRepository eventRepository;
+
+  @Test
+  void saveReturnsEventWithGeneratedId() {
+    Event event = buildEvent();
+
+    Event saved = eventRepository.save(event);
+
+    assertThat(saved.getId()).isNotNull();
+    assertThat(saved.getTitle()).isEqualTo("Test Event");
+  }
+
+  @Test
+  void savedEventIsFoundByEventToken() {
+    Event event = buildEvent();
+    Event saved = eventRepository.save(event);
+
+    Optional<Event> found = eventRepository.findByEventToken(saved.getEventToken());
+
+    assertThat(found).isPresent();
+    assertThat(found.get().getTitle()).isEqualTo("Test Event");
+    assertThat(found.get().getId()).isEqualTo(saved.getId());
+  }
+
+  @Test
+  void findByUnknownEventTokenReturnsEmpty() {
+    Optional<Event> found = eventRepository.findByEventToken(UUID.randomUUID());
+
+    assertThat(found).isEmpty();
+  }
+
+  @Test
+  void allFieldsRoundTripCorrectly() {
+    OffsetDateTime dateTime =
+        OffsetDateTime.of(2026, 6, 15, 20, 0, 0, 0, ZoneOffset.ofHours(2));
+    LocalDate expiryDate = LocalDate.of(2026, 7, 15);
+    OffsetDateTime createdAt =
+        OffsetDateTime.of(2026, 3, 4, 12, 0, 0, 0, ZoneOffset.UTC);
+
+    var event = new Event();
+    event.setEventToken(UUID.randomUUID());
+    event.setOrganizerToken(UUID.randomUUID());
+    event.setTitle("Full Event");
+    event.setDescription("A detailed description");
+    event.setDateTime(dateTime);
+    event.setLocation("Berlin, Germany");
+    event.setExpiryDate(expiryDate);
+    event.setCreatedAt(createdAt);
+
+    Event saved = eventRepository.save(event);
+    Event found = eventRepository.findByEventToken(saved.getEventToken()).orElseThrow();
+
+    assertThat(found.getEventToken()).isEqualTo(event.getEventToken());
+    assertThat(found.getOrganizerToken()).isEqualTo(event.getOrganizerToken());
+    assertThat(found.getTitle()).isEqualTo("Full Event");
+    assertThat(found.getDescription()).isEqualTo("A detailed description");
+    assertThat(found.getDateTime().toInstant()).isEqualTo(dateTime.toInstant());
+    assertThat(found.getLocation()).isEqualTo("Berlin, Germany");
+    assertThat(found.getExpiryDate()).isEqualTo(expiryDate);
+    assertThat(found.getCreatedAt().toInstant()).isEqualTo(createdAt.toInstant());
+  }
+
+  private Event buildEvent() {
+    var event = new Event();
+    event.setEventToken(UUID.randomUUID());
+    event.setOrganizerToken(UUID.randomUUID());
+    event.setTitle("Test Event");
+    event.setDescription("Test description");
+    event.setDateTime(OffsetDateTime.now().plusDays(7));
+    event.setLocation("Somewhere");
+    event.setExpiryDate(LocalDate.now().plusDays(30));
+    event.setCreatedAt(OffsetDateTime.now());
+    return event;
+  }
+}